If she's on medicine to contain her anorexia she knows she has an issue. You could start with simple asking her what she eats and listen empathically.

I would also suggest that you think about your relationship with her. What does she want? Does she want your approval? Does she want that you tell her what to do, to not have to take the responsibility for herself? Does she care about looking beautiful to you? Does she want a relationship with you? Do you want a relationship with her?

Knowing answers to questions like that is important when you deal with deep psychological issues. It shapes how the words you say will be understood.