I kinda doubt the herb is his main problem, sounds like he may have been a bit unstable to begin with. Did you ask him if anything else was bothering him? If he is sick of the herb, then he shouldn't have much trouble quitting it. If he's not sick of it, but feels that it's interfering with his life, then maybe he could try cutting down a bit. (like maybe saving it for the weekends)

Sorry, i don't have a lot of answers.