There's really not a lot you can do for your friend if she won't listen to you AT the end of the day it's her life and she can life it however she pleases - but I think you already know that so I'll shut up about that. How exactly did you try and bring the subject up@? I mean basically, how TACTFUL were you? Because you have to be very careful when you are saying something that might sound like you are telling someone how to live their life. You have to really take the soft softly approach and make them see that you are only even mentioning it because you care about them. The second they get an inkling that you are infringing on their right to do as they please with their own life then they go on the defensive. SO you have to be really subtle - or at least really gentle - about how you approach her. Just start off by telling her you really care about her friendship and she means a lot to you. tell her how much you value her and her happiness and you want to see her happy. Then tell her that you're worried that the way she's been acting lately might have some reason that you don't know about, and that people are talking about her and she's getting a rep. Tell her you care about that and don't want to see her go down that path.
If you can't get through to her that way then try writing her a letter saying the same thing - about how much you care about her nad how much you want to see her happy and you are worried about her. Sometimes a letter is easier for people to read than to actually hear these things said to them.
If none of that works then there isn't a lot you can do. All I can say is try to be her friend as long as you can - distance yourself if you have to but try and remain her friend. For all you know there is something inside her - some feeling or event - that has sparked this behaviour and she's trying to deal with it. For all you know she's going to turn around in a month and realise her mistake and really wish she'd listened to you. So try and stay as near as you can so that if she needs you at some point you're there. And remember - we all make mistakes sometimes, we all sometimes go down the wrong path, and it's our friends we need when we do that to be there and hep us out when we realise what we've done. It really is the greatest gift you can give her.