What should you do if a person confides in you his or her suicidal thoughts and makes you promise not to tell anyone?
Ask what is wrong.
Ask what is wrong, and try to make the person think they should live on.
Tell the person that they would be missing alot of things in life. Give examples. If possible, make the person make a small laugh, or big if possible of course. If you manage to make the person laugh by making a small joke or something you can be assured that you are on the right track.
I do not have any experience in this matter, but I think that this is how I would have done it.
I do know though, that just talking will help.