Wow, I have new found respect for you MMH, just pick better friends next time.
What you should do first is have a talk with him, but don't start off violent or even raise your voice. Try go for intesity and firmness rather than agression and tell him what you have to say. If after that he's still an asshole then you shoulc kick his ass. If he starts going off the handle shouting while having the talk also feel out how you think the argument is going, then beat his ass down if needed.
Your girl obviously is a bit fucked up around this guy and would probably be pissed off if you beat the shit out of him unless you had a reason so I would try and avoid it.