@Joe,
this is speaking from personal experience-
there certainly are mental states that are so painful it isnt worth living-and there is no way of knowing whether or not they will ever end. i dont believe you can go by what any doctor will predict as far as prognosis, if they even will predict any such thing, because one can be changed so completely in one night that they will never again feel that kind of pain and despair and will be grateful for each new day even if it brings sorrow and misfortune. or they may go on the rest of their days with no improvement, the best that life can offer them being sedation and senselessness.
the dilemma is then, suppose you know a person who is in this condition, not just today but has led this kind of mental state for years, and you know he has bought a gun and gone off to shoot himself. do you stop him, or call the police, or just let him go?
i would say that if you stop him or try to in any way, you should be prepared to be responsible for his suffering afterwards, and know that you are the one who maintained it, which is more likely what will be the result as opposed to the chance of it ending some day in the future after who knows how much more pain. hubris will go away, i think.
and if you say nothing and let him go, know that you could have stopped him from pulling the trigger, but you couldnt stop his pain-so what would your interfering have accomplished?
it's a terrible decision to have to make...