Plus, it's a fight over a boy??? Please, let the women duke it out in the arena of ex-boyfriend tension. They'll realize that they were fighting over something stupid and get over it. The man made a self-concsious decicion to date whoever, so the fight is his problem, not yours. It's his place to pick a side, not yours.
All you'll end up doing is giving yourself stress and end up worrying about which friend is gonna keep you and which is gonna blame you for choosing sides. That's why I never interfere with fights between friends of mine--regardless of the fact that they know each other or not.
In fact, once every blue moon, things like this happen; where I'll have to be a mediator between friends. I tell them right away, "I'm not here because I wanna choose sides, nor am I here to tell you what you should do. I'm here to tell you what you COULD do, in order to make things easier on yourself." After that, it's up to them to decide what they want to do. If they want to fight the other person, so be it. I tried to throw some common sense thier way.