Obviously the parents are ignorant of horses and went out and bought the 1st one that came along. I'm guessing that they are regretting the decision and want this girl to also get so discouraged that she gives up and they can get rid of it. This happens constantly in my area where parents give in to horse crazy little girls and buy without doing research into what owning entails, and without providing the kid with at least riding lessons.
They don't want you or anyone else on their property so you have to honor that. However, if the girl is allowed to hang out at your place with you all you can do is have her watch how you handles and work with your own. Explain why and how for everything you do, starting at the very bottom basics like leading, tying, grooming etc., then work up from there. Let her practice a bit but continue to correct her.
Lunging isn't going to be possible since her home area is hilly so you will have show her other methods of making a horse respect a human.
It's obvious she should have been started in riding lessons long before her parents bought her this horse but it sounds like this is not going to happen.
You may have to just back off from your ambition to help this girl and just do the little bit that you can do. Don't get your own hopes up too much.
Chances are that the parents will wake up and sell this horse anyway-they will become disgusted with having invested so much time and $$$$ in a project that this child can't handle.
Sad, but it happens all the time.