What do you do when all of your friends are fighting?

I have a small group of friends, maybe about 8 of us. And everyone of them is arguing with each other. One of my old friends (friend A) broke off from my current group of friends to hang out with a guy she likes, and stopped all connections with us. So because of that my friends started talking badly about her, saying what ever they could that would make her look bad. 6 of them, are in a class together at school, (I’m not in the class, but my sister is, friend A isn’t either.) And all they do is talk about friend A, so my sister (me not knowing) told friend A that they were talking about her, because she was fed up with it and thought it was best for her to know. Well my sister just turned on a bomb by doing that. So friend A now knows what everyone says about her, and is heartbroken by the situation. Friend A started yelling at her old best friend asking why she was talking badly about her, and no one knew who told Friend A. My sister emailed us all telling us it was her, shocked everyone just got in a fight with everyone. What should happen? Is my sister right for starting this? Should we all resolve this by talking it out?