First post here, so I apologize if I am breaking some conventions I am unaware of.
I am currently running a group of six people through Tomb of Annihilation
Spoiler Alert (follwing text may contain spoilers)
The campaign is decently well. The group of players is slightly rockier than what I'm used to, but all in all I'm fairly confident I can have all of my players enjoy themselves.
My problem is with one person.
My group of players is mostly a mish-mash of nationalities, so we play on Roll20, but two of them are real life friends of mine. One of those friends has always told me he enjoys the "game" part of D&D 5e more than the roleplaying, which is fair to me. Not everyone enjoys the same things about D&D and he happens to prefer the mechanics and such. He knows that I include a healthy portion of roleplay in my campaigns, but also like to provide challenging and interesting content that does not involve roleplay.
My first alarm bell should've rung when he purposefully made a mute character (An Aarakocra Monk) so he doesn't have to RP as much. I didn't think much of it at the time because his character communicated through writing and he had agreed with a different player to share a backstory. While this player indeed took a backseat to RP, he was still involved in the game. This has all changed over the course of the game. As time went on and everyone was chatting and having fun, he became more and more reclusive, simply not roleplaying and just being there to play. I have talked to him several times during this period but he assured me he was enjoying himself, so I left him be. This situation did get jarring when the character he shared a backstory with died and he did absolutely nothing in character to react to this.
As time went on, he also stopped talking less in our out-of-character conversations in addition to not roleplaying, limiting his interactions in the game to just making rolls when I called for it, declaring his attacks and asking if he could craft items during travel time. It was also at this point that he unlocked "Stunning Strike" as a monk, leaving him to use it constantly in order to get an advantage in combat. This is fine of course, but limited his interactions even further to just rolling his attacks and declaring he used Stunning Strike.
Finally, this situation came to a head when the party faced Tzindelor, a Young Red Dragon. The party was well prepared and had already done massive damage to her. This player came up and did Stunning Strike as many times as he could. Tzindelor passed most rolls, but critically failed one check, meaning she would be stunned and this epic battle everyone had in mind (it had only lasted a round or two at this point, her health was melting ridiculously quick) would've been reduced to the group just hitting a sandbag on HP. So at this point I made snap DM fiat decision and declared Tzindelor has Legendary Resistance.
Now we can all discuss on whether or not this was an appropriate thing to do (Please do make a side note in your reply if you want to, I am interested in other opinions), but the point is that this player was beyond frustrated about this.
Right after the event, he found the magical Greataxe in the dragon's hoard and refused to give it to the barbarian player in the group, despite the fact he can't effectively use it as his current character.
Other players asked in character if he wanted to give the axe to the barbarian (the other IRL friend) since he could actually use it. He did not respond and just said he wished to keep it out of character.
I did talk to him about this particular axe later, just asking why he wanted to keep it. He said something along the lines of him finally getting some loot. It's true he didn't get much in the way of magical items or upgrades, but then again there's only been two notable magic items in the group so far, an Alchemy Jug and the Ring of Winter. The Ring in particular is incredibly powerful, but I am slowly punishing the player for using it over time. (More details on how the ring ended up with the players can be provided on request). Eventually he did end up giving the axe to the barbarian because he was asked in real life by that character's player.
Finally, I've heard from a different GM from a different campaign that this player has been still bringing up this incident with the legendary resistance to him. Citing I am making him completely useless. While I can definitely understand the frustration at this particular instance, I have not stopped him from using Stunning Strike before and I genuinely believed the fight was more fun for it. I would happily offer some form of compensation, but I have no idea what to do since he doesn't roleplay and only spams Stunning Strike. I'll definitely let him wail on a boss sometime, but somehow I don't think that will really fix much (Confirmation bias is a pain).
I have invited this player for a talk, face to face, in the coming days. Can anybody offer advice on how to talk to this player? Suggestions for how to make him enjoy the game more would also be welcome. I will provide details as requested and provide an update after the conversation. Currently I am considering just asking him what he wants, what I could do to help. If we can't reach any solution, I might gently suggest this game might not be suited to his playing style and offer him the option of leaving. I'm not planning on telling him to leave. If he leaves, it will be because he agrees it's for the best.
I do genuinely hope to come to some sort of solution. The tension this has been causing has not only killed his enjoyment of the game, but is also affecting the atmosphere of the entire group, and nobody wants that.