Therapy. The therapy needs to be with a professional who has extensive experience with the treatment of eating disorders. It may also involve the collaboration with a nutritionist and internist. The patient needs to be ready to do the work required in therapy and be honest about her struggles. It is only when the patient is unable/unwilling to do the work that an inpatient may be necessary.
Severity rules here. The severity of anorexia is a major factor.Chronic anorexia has big medical problems; severe muscle mass loss, electrolyte abnormalities that can cause siezures and cardiac abnormalities, endocrine issues including fertiliity and menstrual problems. Outpatient care is best unless the medical problems become life threatening. Counseling, groups, medication and family support are needed.