You can tell you that you value her friendship too much to offer an opinion in this and the best option for you is to be Switzerland, which is a neutral country. Tell her it's not in your nature to pit people against each other; you'd like to remain friends with everyone and the best way for you to do that is to let the situation get sorted out without you butting into the argument.
It's unfortunate when people try to drag you in the middle of something, however, just keep saying - not my circus, not my monkeys. If you need to take a step back from everyone for a week or two to preserve your sanity, you might consider doing that. Good luck.
