I believe that most people, if prevented from suicide, are glad to have been prevented. You might want to read http://slatestarcodex.com/2013/04/25/in-defense-of-psych-tre...

Personally, if someone told me in advance that, if they ever attempted suicide, they'd like me to not interfere - then I'd accept that. But I have a friend who's alive because she hadn't told me that, and I did interfere, and she's glad I did.