If a player character dies in game when the real world night is just starting, how do you manage for the player to still play if he has no other character prepared?
Have in mind:
There's a time honored tradition of, when a favorite character dies earlier than expected, making a new, basically identical, character and claim it's their sibling or offspring, back for revenge. In a situation where creating a new character isn't feasible re-rolling HPs and reseting gear as appropriate is pretty quick and can get them back in the game without too much trouble, and now they have some additional motivation for their character. You might help them go through and pick new bonds/flaws/etc after the session for expediency's sake. Obviously they were overcome with grief and behaving erratically if you need to explain why they maintained personality for that session but then change for future ones.
Alternatively, play Paranoia where everyone gets 6 clones because you die so often, so it's not a problem at all.
In the future, having a couple of pre-generated characters of appropriate level handy is always useful. Never mind unexpected character death, there's also the very real possibility that someone else will show up and want to "try out the game" (and you want to let them).
Let him assist you as a side GM
In our group we often use that opportunity to have the player of the dead PC help out with the GM'ing:
Deciding on the monsters' actions in a fight, managing their health, playing plot NPCs and so on. They often have ideas to disrupt the group in ways you would never think of, since they are used to how you play the monsters.
In the best case that player gets a taste for GM'ing and offers to be the GM in a future session, so you get to experience the player perspective some more.
I keep a collection of around three pre-generated characters behind the screen. You can find plenty of examples online, particularly at level one, but also at other levels. For example, WotC have a set here.*When a character dies and you need to keep going, give the player a choice between the pregens you have (it's better if you have a range of races and classes). At the start of next session, the player can decide either to keep their pre-generated character or to create a new character.
*Thank you to Raj for bringing this collection to my attention.
Building a high level character tailored to a particular game can take quite a while. But making a simple first level character in DnD 5e normally doesn't take overly long.
There are lots of sources for character stats that fit the rules quickly including pre-gen characters in official books, and fan made ones. You can also reuse the stats from the last character and change the name and backstory.
DnD has all kinds of ways of resurrecting a character within the rules. It sounds like you are the DM, so while it may be a little bit of deus ex machina it would not be hard to have one of those methods conveniently fall into the groups hands shortly after a character died "too early". The way it falls into their hands may mean that there are consequences to using it though. A high level NPC cleric for instance may offer to do it in exchange for a favor that is a plot hook for the next stage.
This is more of a tangent than a direct answer, but it would prevent the problem.
When I serve as the DM I normally focus on plot over combat and encourage my players to develop elaborate back stories. To facilitate that, I outright ban PC death unless it is a deliberately chosen heroic sacrifice. Losing a combat will always come with consequences, otherwise it wouldn't be fun, but those consequences never end the campaign for a PC.
This is surprisingly easy most of the time without even bending the rules because most intelligent opponents will prefer to capture than kill for reasons including ransom and interrogation, etc. If the rules say a character would clearly die then I do have deus ex machina step in. Sometimes, especially in low levels, I will simply declare them injured and unconscious but not dead. In higher levels, the party will often have ready means to methods to raise them. If necessary in DND, I may literally have a deity step in to save them but in a way that the deity will expect to be repaid later.
As long as losing still has some real consequences, I find this doesn't detract from the excitement of combat that much while it helps avoid the issue you raised and also encourages the players to invest in long term story development.
It depends how early in the session and how long the session is, an hour into a 3 hour weekly game is different to 10 minuets into a 6 hour monthly game.
Letting them run enemies or allied NPC's can work well for a shorter amount of time. For a longer time I'd consider takeing 15 minuets to speed build a PC, any problems with it can be fixed between sessions.
What I do as a DM, even if it might be "badly seen" by some, is giving a "plot armor" to player characters when we are in the early part of the gaming session. I don't mean it litteraly as "whatever happend you can't loose" but more in the "ok, if you die now, it just break the session, so something bad happend to you, but you will not die"
e.g. My players are attacked by bandits. If it's early, the bandit won't try to kill the players but put them inconscious (which is, in combat, almost the same). If all my players are inconscious, they loose some of their valuables, or are capctured by bandits and have to escape.
My players know that, because I'm honest with them, but they also know that something bad will still happend to them, so they don't try to exploit that in some way (taking risks, or anything).
In higher levels games, since my players have access to resurection spells, I make the plot armor just protect them from things that could stop resurection spells they have access to (so in very high level, they have no armor at all)
