First of all: Don't worry. Having one player that prefers roleplaying over combat is, at least to me, pretty much the default. I think I literally have never DM'd to a table that didn't have that one Bard or Rogue that wants to sleep during combat and gets all excited when they get to a town and they can talk to people. That's normal and usually everyone can have fun together without much problem.
Comments on Session 0
So, usually, this kind of thing would be talked about in a Session 0. The "problem" with new players is that they are, well, new players. They don't know what they like or not about the system/game, because they have never played it before. It's like asking "do you like this food?" to someone who has never eaten that - or anything similar, for that matter.
And, similar to tasting food, usually tasting it only once is not enough. Even a long session like yours is still little time to get a grasp on what you actually like or dislike in the game. Maybe the player didn't like one very specific aspect of how these combats happened, but can't figure it out yet.
So... play a little more as usual and find out what the actual likes of the players.
Early levels are usually a little more boring than the rest of the game
One of the main reasons to try to keep playing is that early levels in 5e are quite boring. I am assuming you ran this session with 1st level characters, since the adventure you mention start at 1st level. The Warlock didn't get their Eldritch Invocations yet, they know 2 spells and have 1 spell slot. Their playstyle for now, as with most other classes, lacks choice. There is a reason the XP chart makes characters fly through the early levels: they are only to get a grasp of the combat for new players, and to construct your character personality.
Warlock, in particular, is a class that is very... strange. It's like a full spellcaster that has a playstyle more akin to martial classes.
So, just to reaffirm: let them find out their tastes a little longer. Ask them to try out one more "regular" session.
After that, I have seen a few possible outcomes:
- It is not that they didn't like combat, they didn't like their class, or even the way they have been playing the class. I have recently had a Druid player that felt like she hated combat because she was a Moon Druid and most of her combats were turning into beasts that couldn't even talk and attacking and tanking damage. After she started spending more time in her humanoid form and focusing on casting spells and controlling the battlefield, she got considerably more interested in combats. If the player doesn't like the class, offer them the possibility to change it. They are new players, they didn't know what they were signing up for when they chose the class.
- The player prefers roleplaying over combat. That is okay. If they can stand combat, which is kinda necessary in a D&D campaign (more about that later on "D&D is combat-centric"), you can easily balance between roleplaying and combat and everyone has a good time. More advice about it later on as well, on "Finding a Middle Ground".
- The player hated combat. Well, then it is possible that you can't align your tastes, and it is even likely that D&D is not a good RPG fit for that player (again, more about it later). That is okay, it happens, not everyone is going to like the same things, and they don't have to. The player can find tables on RPGs that are more about Storytelling and have fun over there. Or they could even find a very combat light D&D table. But then the other players have to have that same mindset. It is not clear whether this is the case to you and your other players.
D&D is combat-centric
This is both historical/cultural and designed. Monsters have a XP reward attached to them as one of the most notable numbers in their sheet, and there is almost none advice on how to reward things like talking to people and solving problems that are not encounters. The vast majority of class features are supposed to help in combat. Adventures are written in a way that most of the challenges are overcome through combat.
You can certainly play a story-driven D&D campaign, and make it combat light, but the system design makes it way harder for you to do that.
I think this is a needed advice in case you didn't think about it yet: use Milestones to track levels, or any other alternative ways of rewarding experience. This will considerably reduce the focus of combat and tell the players they don't need to murder everything they see to get their XP share.
Finding a Middle Ground
Now, let us assume the player didn't hate combat, and the other players didn't hate roleplaying. There is a similar question here, and you can read it to get some insight. But some things you may do to find a middle ground:
- Balance the real life time spent in combats and in roleplaying in a way that is satisfying. I am not saying this will be 50/50. Maybe Combat has to play a smaller role. You will have to find that out by playing and asking more feedback to your players.
- Allow and incentivize non-combat ways of solving or at least easing combats. For example, the Warlock could persuade some guards in the place they want to invade to let them in, instead of having to fight the guards. I am currently running a campaign with a roleplayer Bard, and frequently he has solved problems through persuading people rather than killing them. Sometimes, he has to fight, though, and then he is more of a supportive character. He is able and willing to accept that this is part of the game and this is what is fun for the War Cleric in the party. He has found creative ways of having fun in combat in the meantime, instead of just "sucking it up" and "accepting" the combat. Your player will probably be able to do the same, eventually.
- Design encounters with meaningful choices. This is extremely important at early levels, when the class features don't provide enough choices by themselves. This will take time to figure out as a DM, but you can read advice, for example, here. Disclaimer: This is a link to The Angry GM. Strong words used frequently. Proceed with caution.
- Make the game about a goal, and combats are a challenge to overcome to achieve that goal. Find out what motivations your player and his character have, and include that motivation behind your combats. Again, talking about the roleplaying Bard: there is a Tome that contains information that he really, really wants. For him, a combat that makes him closer to that Tome is a meaningful combat. Even though he doesn't like combats, in general, that much, the motivation behind it is enough for him.
The last two bullets make an important point: Combats (or, more generally, Encounters) are about a clash of motivations or goals. They don't need to start because one side wants to kill the other, and they don't need to end only when one side is dead.
Again going to the Bard example: consider an encounter where they finally get closer to the Tome, and there are some monsters guarding it. They don't care about killing the monsters, they care about getting the tome. Killing the monsters and then safely getting the Tome is one way to solve this, but also is becoming invisible and stealthing through the monsters and simply running away with the monsters. Maybe if they try to just walk through invisible it won't work because the monsters are very attentive and will notice the Tome missing as soon as it gets stolen and will enter a rampage mode, but if the other characters of the party distract the monsters while the bard stealthily steals the tome, and then they run away as soon as he is safe, that is as solved as killing the monsters, the Bard didn't need to enter combat, and the other players got their combats. In order to not make it trivial, the Tome is guarded by some traps that the Bard will have to overcome while the party is fighting the monsters, who are overwhelming the party. If the Bard takes too long the party will have to run or die before they manage to get the Tome and they were defeated (and, in the case of running, defeat also does not mean death: the Monsters goal is to guard the tome. Killing the adventurers is one way to do it, making them flee is another).
Although I have not run this encounter with the Bard I have been mentioning, I have done similar encounters and the party usually would have fun as a whole. By the end they would be like "Whoof you took too long I thought that Gorgon was going to kill me, I almost got petrified you know?!?!", "Excuse me I had to deactivate a fire breathing statue that burnt my beloved cloak... It was a parting gift from my master okay????", with adrenaline pumping in their blood.
Keep getting feedback
You are learning as well. It is very nice that you asked feedback from your players and that you asked advice here. Keep improving. Unless you have players on a really extreme end of the "likes" spectrum and they can't stand the way the other players have fun, you will certainly be able to find a sweet common ground where everyone has fun. Keep asking for feedback, keep finding advice on how to improve on that feedback, just... keep learning.