I also have a friend that is obsessed with smoking, he's the closest I've seen to a reefer addict. But he als has a passion for pitbulls, he's always telling me about the latest book he read about breeding and such, he can also tell you the geneology of all his dogs going back six or seven generations.
People just get obsessed with stuff from time to time, I smoked every day alot more than four times a day for like two years when I lived with a friend who was dealing(and this was chronic not shwag) then one day I just sort of got bored with it for awhile. I stopped smoking for several months even though everyone around me still blazed constantly. when I started again I smoked much less frequently. you friend will probably go through a similar cycle.
If not just club him over the head, stuff him in a large sack and haul him off to rehab. JK, that's terrible.