I would feel compelled to say something.. I would feel it out. I'd say Hi, and if he looked at me fearfully or looked mad, I would back off. If he looked my way, I would tell him what was wrong and leave it at that; not approach or anything. See I actually feel more inclined to help a handicapped or otherwise impaired person as opposed to a fully functioning adult. Sure, I dislike the fact that some people "dug their own hole" and made themselves into derelicts when they were originally healthy. But my heart bleeds for people who are mentally or physically ill.
I Hate the gang mentality that small-minded people have against the mentally ill. At work, there's a friend of mine who is bipolar. You should hear people talk about her behind her back.. shaking my head. I have much more disdain and dislike for "normal" people than I do for the handicapped.
Now, if this person was a drug addict knowingly to me, or if I could tell through smelling it or seeing paraphanalia (like a crack pipe) I wound definately Not approach or talk to them.. you Never know what they will do, and they are desperate people, and yikes.. I am not talking about your casual pot smoker. I am talking hardcore crackhead.. stay away from them.