I'm not saying that at all. If someone is talking about doing it by all means talk to them, point them in the right direction for help with the numerous charities available that provide excellent services.
What I am saying though is that those who actually go ahead with it, more often than not never talk about it, they just do it. There is nothing you can do in those circumstances because by the time you find out, its already over. All you can do is support those who will be affected by it most.
Case in point, the post made by dave28 above, I'd be surprised if anyone was even slightly aware that it was something the neighbour was even remotely considering. Same with the likes of Robin Williams, a hugely successful actor/comedian who was wealthy with a loving family, and out of the blue with no mention of even considering it, he commited suicide.