^^ What Final said. And there is a suicide hotline that's easy to remember: 1-800-SUICIDE (no, I'm not kidding - that's the actual National Hotline number).
Talking about it and talking through it with her can be extremely helpful. Sometimes people in suicidal situations just want someone to talk to - and it's okay to talk about the deed and what it might mean (just because you talk about it doesn't make someone want to do it; in fact, talking about it can diffuse the mystery of it).
Question her about it and get her talking. And for you? Find resources that can help her. I had a friend a few months back who was in a severe health crisis and he was really planning on committing suicide. We talked for more than three hours but he'd settled it in his mind because he felt so bad and no one knew what he had, and they'd stopped providing the medications he needed for the pain (ongoing deteriorating condition since he was 15). I took the initiative to contact his residence hall and a residence hall director, and I also called his doctor. I also spoke to my supervisor about it since he initially came to see my supervisor for academic advice, and I did a write up for him.
Long story short: He was mad at the onset, but he really appreciated what I did. He's still alive, his doctor fought for the medications he needed, he's feeling healthier, and he has a new woman in his life who's kicking his butt! So all went well.
I can't promise this will happen for your friend, but hope is usually the problem. Someone who is suicidal no longer has any.