I'm falling for my roommate's gf
Will he be mad about it?
Of course he will be upset because I'm distracting his girl.
It is wrong to become involved in a friend's relationship.  It is even worse if it involves cheating.
But I think I can make her happier than he can. They are miserable together.
Maybe, but that's up to them, not you. You need to just leave their relationship alone.