I have... but it's not something I do lightly. I've had to intervene for a few people who have been suicidal, one of which I've had to go behind her back a couple times to let some trustable people know when she's too bad off to ask them for help. Normally she'll go to someone herself if I encourage her to, but every once in a while I know after talking with her that she needs someone to speak up for her.
It's a very difficult thing.