she's s'pposed to be your bestfriend right? then go and talk to her. communication helps a lot. and like what Kathleen said, remind her of STDs not all of them are protected by condoms, if ever she is using any that is.
I had a friend like her before. she was mai close friend during first sem of my first yr in college. she had big tits (sorry if that's too r-18

) and so all the guys were after her. she was quite pretty too, and she looked "easy". she went around kissing every guy she got hold of. But she doesnt "goof off" around me. coz I usually scold her and stuff.. so yah. one time though, during our english class, she kissed randoms guys. I was absent then, and when I heard I got so mad. What she did was really pitiful. I told her she was a disgrace to all females for doing such a thing... all because it hurt me too. Found out she was only "hanging out" with me coz I knew a lot of people in the uni. I thought what she did was too much because I've considered her as a friend.. so I told her to call me when she's done doing the town and ready to stop. she never called me since then. oh well... her loss.
all I'm saying is... there are people whom you think are worth your time, your effort, your advice, and your care.. but they don't. try to talk to your friend... see if you get through to her. if she does, she's worth keeping and caring for. if she doesnt... distance from her for a while and if she comes back apologizing, she's worth it. if she doesnt, move on and find someone worth caring for.