"poetic description" is the only real option from the list that is not common in expository writing, although careful descriptions can indeed be used as long as the point remains clear and forceful.
Approach an adult such as a high school counselor. Inform them of the situation. Then, after the adult consults with the friend, I would approach him/her and help them out with whatever they are going through. I'd tell them that I'm there for them and try to help to get them to stop.
