Is it my fault me and my best friend isnt friends anymore?

Yes you fail?! That doesnt even make sense annnnyways! If this is the truth. It doesnt sound like you fault at all. He has control over her and she doesnt have the to stand up to him. He is intimidated by you because he knows you most likely will tell her whats up and put "bad thoughts" in her head (ex:he isnt good enough, he is bad, leave him) that type of stuff. Good Luck tell your friend goos friends are hard to find