I can tell you what I wouldn't tell them. I hate when people tell suicidal people that they're being selfish if they commit suicide because other people care about them. We must live (or not live) ultimately for ourselves, deeply suffering so that others don't have to see us go would be imbalanced. It's selfish of the people who say that to a suicidal person to expect them to live in suffering just so others don't have to deal with their loss. It's a manipulative ploy. And the most it will probably do is make them feel guilty about committing suicide, as if it weren't hard enough already.

Anyway, as to what I would say, I'm not sure, but I probably wouldn't try to search for the right words to convince them not to do it. Just because most people fear death more than anything doesn't mean suicide necessarily isn't the best thing for them. Bite the bullet and let the poor guy end his misery. (If you really have a good reason to believe his suffering is only temporary, then that's another story. But only if you're realistic..)