...but those are so much fun Xeph... you should try being an animal sometime.

Ren, I've been in a situation like this before. I understand what it is to not want to taint the whole group by snuffing out one who's being less than you'd like them to be. What I did was briefly tell the person what I didn't like that they were doing, in this case it seems like she's sleeping around and leaving herself open to getting some pretty nasty diseases..?
IF they don't shape up, that's when I continue to be friends with everyone else, but slowly fade that person from the group. If someone brings her up, find a way to change the subject. If you're challenged over your feelings than admit that "yes, I don't like what xyz is doing because she's hurting herself in these ways: a)______ b)_______ c)________. I've tried telling her how worried I was, and what could happen if she didn't stop and she's not interested in listening to me; so at this point I'm not really interested in her. She's got to meet me half way." If your friends can't respect that, you may need to take a re-eval on who your friends really are.