I would not dissuade anyone from comitting suicide. I would instead make sure that the person was sure it was what he or she wanted.
In the end, suicide is an option that should be decided by the suicidee, not the people around him/her. Telling a miserable person that they can't kill themselves because it will hurt the people around them, is a bullshit move. A man can't live his life for others. We need to respect the fact that people's lives and bodies are their own, and, ultimately, what they do with it is their choice. Some say that we should prevent people from offing themselves, because they aren't right in their minds if they want to kill themselves, but I reject the notion that we have the right to judge whether one person is sane just because he doesn't share our values or oppinions.
No, I would sit down with the person. Talk things over. I would tell him that if he ends things now, he will never esplore the possibillity of things getting better. Death is the neutral outcome - once you're dead, you don't feel anything anymore. So while it's the escape from pain, it's also the escape from joy. I would say that if they have the determination and means to commit suicide now, why not wait a bit, see if we can clear up this mess. Suicide is always an option, and he or she can always do it later, if they still want to. The knowledge that they can always kill themselves at any point can be a very powerful motivator to keep going, actually, and provide some comfort. I know this, because I've been there.
Now, this is my ideal. Whether or not I can live up to it, is another story. I fully expect that if someone close to me was on the edge, I'd do everything in my power to prevent it, no matter the person in question's wishes. It would be selfish, I'd be doing it for me, so that I'd keep a loved one, but I suspect I couldn't beat my own emotions.
Also, someone on the first page said that an important thing is to be there for people - that physical presence alone can do more than words. I very much agree with that. In the darkest period of my life, I spent a lot of time with a girl I knew. We wouldn't actually do much together, we'd just be in the same room, doing seperate things, not speaking much. That helped me a lot, being able to be aroun others, without any responsibility or pressure to be social.