Very weird... I'd say look up online or in the phone book for a family NUTRITIONALIST. Get their advice. More than likely, the nutritionalist is going to say that it is not good at all. Tell her parents what the nutrionalist said, and be very serious in tone. If there is further trouble, and a doctor has agreed that it is dangerous, get advice from child protective services. Don't do it in secret. Explain each step that you may take to her parents and why.
They are family, so don't "turn on them" (not implying that you are). Help her parents, BUT at the same time the child's health comes first.
It would be GREAT if you knew a doctor well enough to invite them to come over to your house to talk to her parents!
Also, keep in mind that kids DO change their habits, however parents need to be there to guide them to make those choices.
Hope this helps