(Throwaway) A little back story for context. My best friend has been dating this girl for probably near 7 years now. We've all started college at this point and all go to the same college. They used to fight physically when they were in junior high but probably since freshman year she's been physically and verbally abusing him. Now I'm not saying all of it is unprovoked but (in my eyes) she's a psycho. I was supposed to hang out with him the other day but I ended up being way late on schoolwork and had to cancel (fuck finals) and she needed to get a hold of him. He wasn't responding and she knew we had planned to hang out so she messaged me to ask him to call her. (Super passive aggressively I might add) I told her that i didn't end up hanging out with him and she called me a liar. So being the reasonable person that I am I sent the texts between my friend and i with a super passive aggressive message back. She doesn't like it and we ended up fighting back and forth for probably a week or 2 before she told me he constantly abuses her sexually. I told her that I didn't believe her and that she was just trying to ruin our relationship. How am I supposed to, she is abusive to my friend and manipulates everyone she talks to. 

Tldr my friends girlfriend is a manipulative bitch that will do anything to ruin our friendship