I recommend the following:
1- Encourage this person to stay away from friends who do drugs. It’s impossible for him to stop when he is surrounded with such people. Encourage him to find good and responsible friends.
2- Don’t tell him to quit immediately. He won’t be able to. Help him make small goals that are achievable. Every time he reaches the next goal, he should consider it as an achievement.
3- He must be busy to the point where he has no free time. The busier he is, the more capable he will be to stop.
4- Remind him that he doesn’t own his body. God does. His body is simply an Amana, and he is not authorized to do whatever he wants with his body. Allah will hold him accountable for what he did to his body. Harming his body is a violation of God’s law.
5- I recommend this Du’a:
If he is not willing to read it, you read it on his behalf.