First off, I would like to say...
Shame Shame on her parents for keeping her "tied up" outside. That is terrible. and I hate to say it, but in my opinion it is animal abuse. They cant keep her inside?
Second, sounds like she does have another issue besides" being fine". If their is no way for her to get to a vet , their are over the counter pills that can be given for worms. But I doubt that is all that is wrong with her.
If they still refuse to take her to the vet, I hate to say it but I would call animal control on them, because it is ILLEGAL for a pet owner to deny their pets medical attention if it is deemed necessary. And being tied up, isnt right. In no way, shape, or form.
And if you cant call animal control, I would have a serious talk to her parents and tell them that is crulty, and that they should give her to someone that CAN AND WILL take care of her. She deserves it. I hope this helps.