"Good in moderation." This applies to EVERYTHING in life.

Food: Humans need it to survive, but eating too much makes you fat/obese and leads to other health problems.

Water: Humans need it to survive, BUT, and I never thought it was possible, someone actually died from drinking too much water. Story in this link --> http://www.myteespot.com/images/thumbs/t_6583_01.jpg

Drugs: Helps people cope with pain and/or stress. Everyone needs that outlet, but too much will kill you, or just fuck your life up and do some permanent damage.

So tell your friend that he has a problem, and help him control it. Be accountable for your friend. Share some of your hobbies with him and see if he's interested, then work from there. Or just cut all ties with him. Either one will be good for you at least. Be careful if you don't try to help him though. He may try to rob you or your house. I know a guy who broke into his ex-best friend's house and only stole stuff that was his friend's because the thief remembered a time when his friend pissed him off. This thief guy is almost exactly the same as the person you described, except he loves weed and every other drug made.