I think that the details of him brilliantly coding while at the Ballmer peak is kind of irrelevant. It can be a huge liability to have people intoxicated in the work place, and I think it's important for your manager to have an initially undocumented, private chat with him about it.

In alcoholism, an important concept is 'enabling', where you are making it easier for the person with alcoholism to continue with their habit. You are enabling your co-worker to continue drinking by giving him rides home, having interns walk him to the correct apartment, and allowing him to commit code while he is clearly under the influence.

Your manager should be setting the boundaries in the workplace regarding alcohol, but the best thing you can do as a co-worker is to be there as a friend and to listen. Take a break from the screen and step outdoors and have a conversation, because it sounds like this person is really struggling with a difficult chapter in his life right now.