Telling her to quit what makes her happy might make her more depressed. What about sending to visit a relative? She needs to stay away from the instructor. I fear this is one of those instructors that push models into anorexia.

Advise her to take a break and work on her mental health. Your daughter is beginning to hate her body is a real issue you shouldn't neglect. Oh my God! I feel bad for these girls the instructor is training.