Depends. If one has a just reason for it, then yes, of course. Besides, even if the person in question didn't have a good readon for wanting to kill themselves, they still have the right to commit suicide. Their life, their choice. Sure, they have the right. But I wouldn't let them do it, either way.
Hypocritical of me as it is, I wouldn't let my friends, family or anyone kill themselves if I were there to stop it. I don't know why. Inate instinct to preserve life, I think? I'm very much convinced of the mindset that there's always hope, that there's always a better way. And I wouldn't want someone to take the chance of things getting better away from themselves by comitting suicide.
Anyway, it doesn't matter whether or not it's illegal. Suicide is one of those things you can't stop by outlawing it, because everyone knows that if you do it the right way, you're not going to have to face up to it and handle the consequences; you'll be dead, after all. What use is there in making a crime illegal of which the committers know they won't be there to do the time?