Her trying to disavow the fact that she interceded in his tantrum by telling him he sucks at the game is *insane.* If you're a gamer you should know how cruel of a response this is!
So I guess it sucks to inherit a kid with behavioral problems. This is a pretty mild tantrum to be throwing, though. Easy to redirect: ask why this particular game, is there a different mode to try, can you find another group to play with. Rather than siding with his opponents.