Emily, this is my opinion, for what it's worth.
What you can do: listen, talk, be a friend. Strongly advise her to get professional help. Call suicide prevention, call a psychiatrist. If someone you loved was running a very high fever, you'd call a Dr immediately. This is as severe an illness, only mental, not physical. Mental hospitals and current psychological treatment may not be that great, but it's better than doing nothing. If she's religious, she could talk to her priest/rabbi/minister/???
What you can't do: give her the will to live. You can do everything within your power to point out reasons for her to keep hope alive, and show her she's loved, but she's the one who has to want and accept those things. There are no magic words.