Just tell her you love her and that you don't want to lose her. It is okay to talk about the elephant in the room. Tell her you are sad that she is an addict and that you hope she gets help.
Then next semester you must try your hardest not to let her addiction interrupt two lives. You focus on your studies and be the best that you can be. She is young and it may take a long time for her to come around.
You must keep your joy and ambitions your focus.