As for an approach to this all, weening down works fine but it might just be best to go cold turkey. Weed abuse sort of comes and goes...after some time you probably won't miss it much at all.
This sounds like the sort of drivel that non addicts say, like "just quit man" and "don't be a pussy". However, with cannabis I can confirm this is absolutely true, at least in my own experience.
Cannabis was like Heroin to me, I was utterly dependent on it. When times were dry I've been known to drive 200 miles to pay double price for a couple of grams. The idea of going without terrified me.
When I actually decided to quit, despite YellowLights' advice I actually kept a little bit of green in my tin, just to avoid the fear, though this might not work for everyone. After 24 hours I felt like "this isn't so bad" and after about three days I was cursing myself for not doing it years ago. It turns out the fear of quitting was FAR outweighing the reality. About a week later I chucked the emergency supply away. In order for this to work, however, you have to want to quit full time.
One more thing - does he smoke it with tobacco? The reason I ask is because I did, but I rarely if ever smoked cigarettes without cannabis in them. I think it turned out for me that a nicotine addiction which I was in total denial about was masquerading as a cannabis addiction. Sadly this meant when I quit the weed I became a full time smoker, but I have since cut this right down and switched to e-cigs and hope to be done with them by Christmas. If your friend smokes it with tobacco, I'd say give up the weed first, then work on the tobacco separately, as nicotine is a bitch to give up.
Cannabis is a wonderfully enjoyable drug, and unlike most drugs if your friend quits the daily habit for a year or so then he might be able to go back to getting high once in a while - I know I did, I now get high once or twice a year and its very pleasant. However that's just my experience, and I apologise if that is in fact terrible advice.
Best of luck to you, I hope something in the above helps.