This is likely to have gone on for longer than she's been an adult. Try gently asking her what her childhood and teen hood was like. When the pressure to be perfect and thin started. And consult the feeding clinic for advice on what your legal rights are in helping this girl, her rights and the parents rights and obligations. This is to protect yourself from legal backlashing. Seek an attorney. Contact your local police department and tell them of the situation.
It sounds like the parents are not only neglectful, but this could be a case of cruelty, maybe even slavery.