She's an adult and the choices she makes are her own. It's okay to tell her how you feel and ask if there is anything you can do to help. It's okay to refuse to buy her snack cakes and invite her to go for a walk/work out with you with you.
Beyond that, no matter how much you want to, there isn't anything you can do. She is legally competent and responsible for herself. It is her decision and yes, it would also be her decision if she were using drugs because she is an adult. Even if you were a professional, you wouldn't have any business taking her on as a client because your relationship is too close for you to be objective.
If we are incurably at odds, it has nothing to do with type because I get along with my INTP husband very well -- precisely because respecting the autonomy of others is one of my cardinal rules. It's about respect and not setting yourself up as some kind of superior authority who has the right (moral obligation?) to control them for their own good.