How to talk friend out of suicide? This is urgent.
Let him do it.
>Are you in crisis? Call the National Suicide Prevention Lifeline at +1 (800) 273-8255.
First accept that there isn't anything you could say that would heal them instantly. Curing depression takes time. You can't change their mind for them.
So if they go through with it don't blame yourself for not "finding the right words".
Next counter intuitively you don't have to talk at all. Listening could be just as helpful.
http://www.cracked.com/personal-experiences-2338-5-disturbing-things-i-learned-working-at-suicide-hotline.html
Also this
http://www.cracked.com/personal-experiences-2496-meet-cop-whos-saved-over-200-lives-and-counting.html