Edit: I am pro euthanasia, but it should be discussed; most cases are fixable, the ones which aren't well aren't.
@erre below (I cannot reply): we agree, the moment itself definitely not rational (that's probably why so many people actually change their minds during the fall from the Golden Gate; there is a good documentary about that), however during the years and years of getting to that point there are rational moments. Your friend went to seek help ; that was rational. It does not have to be continues; it's just sad to see how many people do this without ANYONE actually knowing they even had anything like that on their minds. My cousin was an upbeat, happy guy; he was always the most cheerful and nicest person you could have around. He had tons of girls around him and a steady girlfriend. He went for a drive, cheerful as ever, from his house, left a note + a tape for the funeral. No-one had a clue. A forest ranger found him in his car. That's just weak, really, especially considering his note explained the years he spent coping with this and that he couldn't take it. The moment itself was not rational by any means, however the YEARS before and the show he put up was.
Except that I have close contact with someone who suffers from depression, and I know it's very hard. She sought help, and is much better. But, during crises, none of this matters. You cannot argue with them, try to get them to see reason. Cause and effect just don't get through to them.
I know it's exasperating. My initial reaction was to get angry, to tell her the same "you know this and that", until I realised it was completely beyond her control. I'm very, very happy she got better, but I fully realise that if she had jumped out of a window at some point, it would not have been a rational decision; it would not have been "irresponsible" (the concept wouldn't even apply); and she would have sees absolutely no other course of action she would have been capable of having taken. It's very sad, and very hard to deal with.