Be present for them. Just having a friend present in my life literally stopped me from ending it. In my case I remembered the feeling I had when a girlfriend had tried to kill herself a few days after a date with me. I didn't want to cause that pain to my friend. This gave me enough pause to get rid of my means of suicide and go get help. Be open with your friend, but don't pry. If they don't want to talk about something trying to force them to may only cause them to push you away. If you've had any issues (physical, mental) that you haven't discussed with them, considering opening up to them about it one day. If they know that you trust them with your issues, it may help them to talk more openly to you about theirs. This isn't 100%, it doesn't work for everyone, but finally having a close friend I felt I could confide in helped me turn myself around.

I hope things work out for both of you. And please understand, whatever decision your friend makes is theirs. You can only help them so much.