while i agree with confronting an issue that you and others might see getting out of hand, i don't think it's a good idea to speak for other people, or tell someone what other people might have told you in confidence, without their permission.

Agreed. Let others in your group of friends know your intentions, but don't let people too embarrassed or cowardly to act keep you from doing the right thing and bringing it up.

yeah, this was another issue. I'm apprehensive of involving other people in a passive way (saying "I've also heard that person A, B, and C, are also all really concerned about you"), but I think that it lends some gravity to the issue if he knows its more than just me. But to try and involve these other people in a more active way then seems like an "intervention" which feels more public and is more likely to make him react defensively...

so mixy, you've already begun talking to him about it? How has your initial approach worked out so far, has he been receptive?

I guess I don't know much about egos and internalization...
The first red flag was when this coworker slipped out the back door of the bar at happy hour and attempted to drive somewhere. When the rest of us realized he was missing and repeatedly called him, we launched into a search and rescue mission that culminated in me driving him home. Next time I saw him at work, I said, "it's fine to drink, but you had no business driving and you're damn lucky you didn't get pulled over or in an accident." He apologized, but I got the feeling I was too pissy about it and maybe not understanding enough? Anyway, I confronted him alone, but obviously others where there and knew what happened.
Another incident happened at a different coworker's house. He had planned to get a ride home, but later stopped answering calls and texted scary messages, like "HELP," to several people, some of whom then called me freaking out. I told his girlfriend and brother I would address the issue with him, and when I did I reminded him about not wanting his actions to ultimately do harm. I also said I and others were paying attention to these behaviors, and I would listen if he wanted to talk about anything that was bothering him. He apologized again and said this time that he would apologize to the people who called me in a panic. I guess I just wanted to let him know that he wasn't alone or being ignored in this issue (for better or for worse). He hasn't come to me about this any further though. We're not super close friends and I don't want things to get weird because we share an office. I just tried to be really serious and forthright on the point, but keep it out of the overall working relationship.
Anyway, good luck in your talk. I think your instinct to talk one on one is good. Just think of it as getting the ball rolling, so he will recognize he's got some issues to sort through, either by himself or in some therapeutic setting.