At first your child's new habits seem normal, even admirable: a diet to shape up or new health-conscious habits. Then it seems to be getting extreme: refusing to eat what the family is having, and questioning every ingredient. One day, you realize this isn't a phase, this is an eating disorder, and the eating disorder can get very ugly, very fast. As a parent, it's important to support your child and not to demean them for their mental illness. It's important to separate your child from the eating disorder.
treatment
It is not hard to find someone willing to treat an eating disorder patient. Most therapists, social workers, doctors, and dietitians, with no particular specialty in the topic, will accept patients with anorexia, bulimia and other eating disorders. I've heard of a chiropractor, a massage therapist, and an astrologist taking charge of this life-threatening illness as well.
As parents and loved ones, I think it is really important that we do everything we can to make sure patients get care from clinicians who specialize in eating disorders. Further, it is our job to make sure these specialists are using evidence-based treatment methods.
It sounds like a reasonable question: "What is your ideal body weight?" But beware: this is a technical term that is often confused for what it sounds like: an aesthetic ideal.