If you like him, try to help him, including telling the wife.
If you do not end the friendship, because it will only come back to haunt you.
Either way, inform the wife.
As for those of you saying addicts knew what they were getting into before they started, I suggest you realise that is not the case. Many start as naive kids trying to fit in. Then end up hooked on something. Use heroin as an example. It is horribly addicting, and phenomenally painful to be withdrawn from. Now tell me someone you know that would willing wish that upon themselves.
Drugs, in any shape are not healthy for your body, mind, or spirit. No matter what anyone says, they are not. Remember, anything, that physically and / or psychologically addicts you can not, and is not a good thing.