Why would you do that. They don't need help because it's not physically addictive. If they don't want to stop then that's pretty much it. What problem do you have with them smoking. Maybe if I knew why you wanted them to stop I could help more
First understand you cannot change a person's habit he has to accept that he needs to change. You may try to find out why he smokes weed. Ask him if he knows the consequences of smoking weeds. Try to show him how reliance on the drug is a problem, and his problem will still be there even after smoking weeds. Let him know that you do accept him for who he is and how you feel about the drug. You can suggest other ways to have fun. Never get tired of telling your friend to stop smoking weeds. See more details at http://www.wikihow.com/Help-Someone-Overcome-Marijuana-Addiction.