My bestfriend in the whole entire world is a Slut. What do I do?

Ok so me and her have been friends ever since were were in kindergarten and we are not 17 and 18. We have a really, really close relationship to were we consider each other sister, lol even though we look anything alike. Whenever I need advice she is always there, whenever I need a shoulder to cry on she is always there, in defense, and everything. The only thing is and I hate to say it she's a really big tramp, she always has to have a boy and she is really pretty and has a killer bod that every guy wants to get into her pants and that's basically all she uses them for. She will like meet a guy within a week they are dating and she F's around with them. She just recently started dating this guy yesterday, and she already gave him a bj and let him finger her and what not. Plus she just broke up with her boyfriend of two weeks and she did the same thing with him except she actually slept with him. She does things like this quite often. People at school spread rumors and talk about her to the point were almost no girl likes her except me, but she annoys the hell out of me when she always has guys flaunting around her and using her for . It just bothers me because I feel that we as women should have great morals and respect for our body, I even told my boyfriend I'm all about abstinence. I'm just tried of going through the samething. What should I do.