I know you feel... I've had a friend try to kill herself. it's sad, but the only thing you can do is try to be positive in the darkest of times, no matter how hard or hopeless it may be. Just try to support your friend and if you must, hang out with her more and try to find the reason why she's doing this. If you already know, try to stop it, unless it's impossible.

I hope all goes well in the future!