When another person wants to make a choice that will end his or her ability to make any more choices, I think it's legitimate to intervene.

Examples: Joining a cult, committing suicide, doing drugs on a scale that will destroy cognitive function, go off anti-psychotic medication ... sheesh, looking back at those situations, the only one I haven't been through with a friend or relative is the first.

Anyway, the problem with decision-ending decisions is that the person who goes that route denies himself the chance to wake up later and say, "What a complete barking idiot I've been." That's why it's fair game for friends, family and even strangers to intervene.