That's so sad, hun
If I were in your shoes, I'd never leave her alone. Let her know that you love her, and that there's no use in killing herself since it won't solve anything. Tell her that you will always be there for her, n that you will help her to go through the hard times.
Friendship is one of the worthest things we have, and one of the strongest reasons to keep on living. If you show her that, I'm sure she will reconsider her decision.
Just, stay with her,and remember her that you love her, and that if she ends with her life, she will be killing a part of you too...
my best wishes for you and your friend!!
XOXO