How can you help a person to stop smoking marihuana if he's been doing it for years?

Stop marijuana. You cannot stop someone from smoking marujiana unless they can be convined they want to quit. That is why they need professional counseling and they may need addiction rehab programs. They need to get the addictive substance out of their system and find other ways to cope with the streessors that lead to using the substance.
Mutual support helps. You might consider attending a meeting of marijuana anonymous.