Yeah do nothing. A friend needs to be accepting of your other friends or at the very least loyalty to your other friends. Now you can totally council the friend that started the fight. Let her/him know that it wasn't cool to put you in that position.
Tell them to finish their fist fight and you'll be friends with the winner.......8) Or you could just tell them you don't wanna get all entangled in their silly little drama.....and then tell them, when they are done acting stupid then they can come find you....
walk away and avoid things until everything gets worked out between those two i would just be fair and avoid both of them so they don't think your choosing and just stay out of it hun you don't need that stress and good friends wont make you choose between the other.
Since they asked for your help, you could ask them if they would like you to be a mediator for them. As mediator, it will be your job to outline the rules of good communication for them to follow. This way they can talk out their differences, instead of fighting them out. For mediator guidelines in communication, go to http://www.commongroundmediation.net/divorce/communicationD.html. Good luck.