All jokes aside, since I can't think of any that haven't already been said, I'd ask them why, and depending on his answer, either tell him it's not worth it, or leave it be. If he said something retarded like his girlfriend broke up with him, I'd tell him that killing himself over it was stupid. But if he said he didn't want to keep going to a job he fucking hates just to make ends meet and he's getting too old to wait for it to "get better," I'd probably say... yeah. Just make it painless, and if you've got anything left to say, write a note to a close friend.