Of course you wouldn't tell people at work that he is there because of an overdose, would you? <worried> I don't think that legally you are allowed to divulge that information.
I'm sorry there is so much trauma for your friend. It sounds like you were there for him like a rock. It hurts to be so sensitive and in tune to other people's pain - they are drawn to you for nurturing, but that means that you are exposed to and absorb much suffering. Remember to find someone to hug you a LOT! and to let go of what you cannot change...
gentle hugs