Last year, I was in a friendship with a girl like that, but it was more or less "my boyfriend dumped me! I'm going to kill myself" and what not. I was always like "you got so much to live for!" and all the sweet talk.
And after about 10 times, it finally hit me that she wasn't going to do it, so, the next time, I just said "Do it then, if you're greedy enough to talk only about your problems to everyone else, go ahead. Apparently the world is better off without you if you say so" and I cut all communications with her..
Saw her yesterday, she's doing fine.
I have absolutely NO clue what will happen with you and this girl, but she was kind of in the same boat, she had been raped by her step dad, IIRC, or so she said, and she had depression problems. A crazy bellybutton sister, and a mean momma didn't help much at all, but I gave her a reason to live, for me to be wrong. And surprisingly, I'm actually kind of friends with her again. And she's doing fine. Sis is out of the house, and I think she lives with her dad now.
And that girl right there (Linsay) was pretty out of her mind.
If you can piss her off, and give her a reason to live, give it a try, if not, and it's too hopeless, I'd just try being nice for her, and being the brother she never had. If she really isn't like Linsay, where it just more or less turned into greed for attention, then, hopefully you can work it out.
Only time I can sorta relate. And I did right I guess. Kind of sad though, she has the scars on her wrist that'll last the rest of her life.
I really hope I'm not telling you to do the wrong thing, but I was about your age when this happened to me.