I just know how many times I've been so close to doing it myself, how the world can seem just so pointless and cruel and without any worthwhile merits and how I just didn't want to be here anymore... People who haven't felt the crushing power of true depression don't know what it's like and have nothing to compare it to.
Luckily my very deep preconditioning against suicide stopped me, just barely.
And now the world is a very different place, or rather my perspective is very different. Life isn't all sunshine and moonbeams and sweet smelling roses. It's something else, some mystery there, something under all the pain and hardship (and the good times too) that is slowly showing itself. There's a sense of amazement and fascination that wasn't there before.
Of course, saying any of this won't change the mind of someone who is completely committed to suicide. It wouldn't have stopped me if I'd ever decided to take that final step. I would have reacted like, "well, that's fine for you, but you obviously have a naive view of this world. You might be satisfied with the simpler ideas and explanations of things, but I see things you can't and I know things your brain must not be able to process..."
That's how I would have reacted. I'd have been wrong though. I still know all those things I did when I was depressed, but now I also see things differently. When you're depressed you think that undepressed people are only that way because they are too dumb or ignorant to know what's really going on. And while a lot of stupid people aren't depressed for that reason, a lot of people who aren't depressed are like that because they know more, not less, about the world.
By the way, I haven't "found God" or read The Secret or anything else like that. I've just been slowly chipping away at the world all these years, and now, when I stand back, I see a shape starting to form that wasn't there before.
So yes, I'd try to talk the person out of it. I'd hate to see someone go to all that trouble in their life and then fall at the last hurdle.
And that concludes my sermon for today. 
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IN GIRVM IMVS NOCTE ET CONSVMIMVR IGNI