People thought my wife was borderline anorexic in high school (5'8" and 115). She wasn't. Her father was 6 foot and 140 pounds in high school. Her health was never in danger, and she outgrew it. 12 years later, she is still 5'8", and 135
How tall is the girl? Does the family have history of being very thin?
If the family has a regular physician, I would consult with him/her. In my limited experience, a family doctor will oftentimes know how to approach mental health issues if they have a history with the patient, and will also be best equipped to diagnose any physical ailment that may be a cause or contributing factor. Most good docs also will know where other resources are to be tapped to help the person. A balanced approach to the mental, physical, and spiritual health of the person works best. Be careful that the Doc doesn't just want to prescribe a pill though, I have seen one or two that take that approach, and I personally don't agree with it. Also, being so thin (if way too thin for height/build), there may be physiological issues in getting back to a normal weight.
My $.02
Hope this helps. Perhaps Medcop has some insight?
Edited to add that you obviously plan on looking after the spiritual needs of the girl and her family.....forgot to put that in. They will need you!