Answer:
I once had this friend, S, and we had been friends with each other for a long time. One summer we had another girl join our friend group, B. I wasn't going to exclude B so I was nice to her and we became good friends. I wasn't spending any less time with S, but she still didn't like that I had a new friend, so she started causing problems. She would start fights with B and accuse her of things she didn't do and she would just attack B for the smallest of reason. We eventually said it was enough and told her that if she didn't stop harassing B we were going to cut her out of our friend group. She didn't stop and that's what we ended up doing. I definitely think we made the right decision and I dont have any regrets. She still acts like a child to this day. She blames the world for her problems and refuses take responsibility for her actions.