If you had a friend who smoked or drank or did drugs a lot, how would you get them to quit?

a few of my friends have tendencies to get out of control. one in particular used to be really addicted, and i’m afraid constantly that they’ll get like that again because sometimes they mess up. they want to quit, but can’t find the will. i want to help so bad. i hate all that stuff and can’t stand to see my friends regretting things and messing up their lives.