I am really worried about my promiscuous friend she is thirteen?

I have a friend, lets call her "Sally", and Sally is 13 and so am I. Sally has with this creep named Bernard. Bernard has had with about 5 girls and he is 14, and they had when she was 10 and he was 11, and now she is branded as a slut. And then, my friend told me she was having with her 18 year old cousin when she was 8 years old. Sally gets in fights often, and smokes crack and marijuana. I am very worried about her. I dont think her mom knows about this. I wish I could tell her mom, but then Sally wont be my friend anymore. What should I do?