She/He will most likely not appreciate your help to start of with but after a while, they will.
You have to involve someone of trust and responsibility. Usually parents are NOT a good place to start. Teachers at your school will be able to help, confide in them.
You would feel very guilty if someone happened to your friend and you didn't try to help her by involving someone else.
It is hard and i have been in a very similar position. I wish i would of acted before it got very serious. I assure you that telling someone is the right thing. It does get hard on you as well as your friend. Her/His woes will soon get you down. You will have to stay strong, for your friends sake.