It is not [...] strictly against the law to bring her to a doctor. She can refuse treatment while there though. However, as you are a Doctor (I presume of the non-medic industry, but this might be good enough), this can be waived as she is need of help. This would not strictly be abduction. However, I'd like a little more information if at all possible.
Who is this? Do you have proof that she's not eating? Anorexia and bulimia fall under mental disorders and would have her movement to the hospital more legal.