If the friend is not listerning to our words then we should try to concern his or her parents and join them in drug rehab association. they will surely cure the friends drug addiction very easily and gives a fresh life
The person who take drugs are generally found to be very depressive and weak persons.They have to be tackled very carefully.I would inform his parents about the problem and then try to convince them for admitting him to drug rehabilitation center.There would be experts in the center who can help him in overcoming the problem
We can try our best to explain the darker side of it to him in a mild case and if the problem is a more serious one appropriate professional advice should be sought!