Alright, as a person that frequently combats extremely heavy depression, and has gone through a very long period of time (years...) just constantly think about suicide, let me tell you: Do NOT, under ANY CIRCUMSTANCES, try to invalidate his feelings. Telling him to his face anything along the lines of "Life is worth living" (Oh yeah? FUCKING PROVE IT!) "Suicide isn't the answer" (It's a good enough answer! I've been fucking thinking about it afterall!) "Think of your loved ones!" (What loved ones? They wouldn't want me living like this anyhow, so fuck them!) "Your life can get better" (MY LIFE?! What the fuck do you know about it?! FUCK YOU!). Remember, that a suicidal person's mode of thought is something along the lines of "I can't control where my life is going, but killing myself is taking the control back, and no one can stop me!", and I know this, because I've contemplated it for years now.
The best advice I can give (besides getting him/her professional help) is to just listen to them. Always take them serious. Don't cast judgments. Be there for them. It doesn't matter if you don't understand their feelings, just so long as they understand YOUR feelings, so tell them how sincerely you care by listening.
Just trust me on this one, it's very hard to understand the thoughts of someone that looks outside, and doesn't see a future for themself in the world they live in. That the friends they have are inconsequential to ultimate happiness. That their life to this point has been wasted, and is careening towards an end they honestly don't want to see.
Truthfully, it's not something you can understand, it's only something you can experience.
Lastly, to paraphrase something depressing I read once, "Don't pity those that kill themselves, because, ultimately, they got what they wanted afterall."
Do you understand?