How do you convince someone to Quit?

why not ask her why she does drugs in the firsdt place. and then, if you get an answer, try to talk to her about it and be there for her so she can help quit. if that doesnt work, then get help i guess. if none of that works, and she says no, ask only three times, and then say "I respect your answer and won't bug you about it again. But please don't let these drugs take over your life. Try to take less and less each day until you manage to quit." Hope this helps!