Unless I'm missing some key piece of information about this incident, it seems like you probably should have stayed out of it. Those girls probably weren't going to get their asses kicked.
That's not really the issue, though. The issue is whether you let a friend who is in no way in control of their faculties get taken advantage of by someone who isn't concerned for the friend's safety.
This wasn't his friend. He says he knows the girls who are drunk-girl's friends. Based on his description, an acquaintanceship might be too hyperbolic a term to describe their relationship. He doesn't even claim friendship with drunk-girl's friends.
We don't know that she was being taken advantage of. Was meathead just as drunk as drunk-girl? Did drunk-girl want to go home with meathead? Why do you assume meathead had no concern for drunk-girl's safety?
With this information omitted, I am compelled to suggest that it was not worth getting one's ass kicked over.