Thanks for your input. For the first time I used pre-generated characters, and let them change a few things. The problem was that the pre-generated backgrounds for LMoP are not connected to each other, and this may have created the problem.
Yes, I could see that. I recommend using
the character bonds from Dungeon World. Ask each player to establish one bond with at least two other people, drawing upon whichever ones they like. Then ask the player to whom a bond is established to elaborate on it to flesh out the history behind it. Once you've gone around the table, you've got a ton of context to draw on for how and why the characters work together.
The player in question is actually playing his alignment decently, but I think that the other players are not, due to their respect for the guy. In my mind, the moment a party member steals something from the group, I would probably want him off the group or I would be the one to leave.
There is no obligation in the game for players to play their alignment, traits, ideals, bonds, or flaws. (Inspiration can be used to reward this, however.) In any case, as I said above, I don't think it's cool for a player to do things that annoy other players and say, "Hey, it's my alignment" or "That's what my character would do." That player has, at least in that moment, ceased being fun to play with and that's pretty much Job One when it comes to playing games with other humans, right?
At one time, the wizard of the group (which do not like the rogue, in game) got into a really bad situation with an owlbear. The rogue literally laughed and did not join the fight. If I was the player, I would never accept working with a partner like that. But that would probably mean bad feelings off-game.
These guys know each other for more than ten years, and I think that's the reason why the situation did not solve itself in-game.
I can't recommend trying to deal with what is an out-of-game problem with in-game solutions. If the player is acting in a way that is not fun for other people, the recourse is to the player, not the character. Be direct, be polite, be clear on what you want, and ask for that player's help to achieve it. In-game solutions often result in hard feelings, get-backs, and escalating tensions. It's not worth trying it, in my view, since you can just resolve it directly with a conversation between friends outside the context of the game.
Good luck!