Wow that was unnecessarily long. heh. Anyways, how did mandy get sarah's number?
My advice to your question though, is to forget about her. If she wants to hurt herself that is her business. That may sound a bit harsh, but I've learned you simply can't worry about everyone, and in this case it doesn't sound like there's anything you can do. I mean, you can't date her again, since when you inevitably broke up again it would be worse. What else can you do, go to her house and watch her 24/7 ? Only thing is to try and explain one last time what happened. Be mature about it.
Oh and not that you asked this but, dont let sarah get away just because of some stalker ex.
