Oh my. I've been through this several times with friends. It isn't easy and you'll pretty much have to walk on eggshells.
Be her confidant. If she's telling you these things, that's what she's looking for. If she were going to commit suicide, she would have already followed through with it. The fact that she's telling you about her thoughts shows that she's feeling alone and is only contemplating suicide. Most people that I've dealt with that confide on someone the way she is won't commit the act. With suicides, it tends to be more spontaneous. They didn't confide or seek help- they just did it.
Listen with open ears and try to tell her the wonderful beauties of life. There are people around her that I'm sure care if she were to do something as drastic as that. Try and help her find a long-term goal and help her to achieve it. Figure out her interests or help her find new interests. If she's into music, either show her bands that have upbeat messages or look for a different perspective in the messages of bands she already listens to.
It's a difficult process but, as long as you show her that you care, that's one more person that she thinks cares for her than before.