Boyfriends girl best friend is so mean Update

My boyfriend has a girl best friend who is so rude to me and him. She's made me cry a million times. Talks shit behind my back. Has started rumors that I've cheated on him.... All that. She's also rude and mean to my boyfriend who is supposed to be her best friend. He puts up with her shut because he feels guilty about something that happened when they were FIFTEEN!!! He stole his mom's car and they went out drinking and she opened the sun roof and said she wanted to try a trick and got on too of the car. She road bulls so she told him to drive the car all the way up to 50 miles per hour and she's gonna hold on to the sunroof. She was and the got hit in the face with a tree branch. And fell off the car. Breaking her legs, ribs, arms, it was bad. She walks with a limp now, but didn't press charges on him. We are in our 30's now. If she is still hung up on what happened when they were 15 why not just end the friendship instead of making him feel bad about himself. Bullying his girlfriend and using what happened as a way to guilt trip him into doing her bidding. I did try to talk to him about it and this was his response

@Jen I could try the compromise thing. Here's the deal. This girl knows that it was mainly her fault she got hurt. That's why no charges were pressed because she knows she chose to get on top if the roof. She also knows he blames himself and she uses that to milk it. He was at one point thinking about starting therapy but she was saying how therapy is for crazy lunatics. Any therapist would tell him how toxic this friendship is!

We found a compromise