Your good friend Monica has started using drugs, and you're worried about her. How should you try to help her quit drugs?
Tell her you're concerned for her, and describe her behavior when she is under the influence to make her see that she should quit.
Tell her you're concerned for her, and say that you will have to tell her parents about her habit if she doesn't stop.
Tell her you're concerned for her, and offer support by suggesting specific ways to help her stop using drugs.