Two things I can think of right away:

What food has your friend been giving her bunny that now she won't eat? Pellets are not the best food for a bunny....they need dark green veggies (NO iceberg lettuce!!!!!!!) like romaine and red or green leaf lettuces, endive, dark green outer leaves of a cabbage (not the pale insides), parsley, dill, swiss chard, dandelion leaves, a limited amount of kale, mustard greens or turnip greens (my bunnies won't eat the mustard or turnip greens). They can also have a little bit of broccoli but not alot.
A carrot with it's top is a great treat all bunnies love. They can also have a very limited amount of fruit....3-4 blueberries, or a tiny slice of apple, a strawberry..... Too much sugar can give a bunny wet, messy poo so be careful with the fruit. They should have unlimited amount of timothy hay to help their digestive system. Now I realize this all costs money but I think you should be aware of a proper bunny diet. It is a bad sign that the bunny is not eating and she really needs to get to a vet asap. Since the parents won't pay for the vet you might try any of the above foods and see if she will start eating again.

Many bunnies get arthritis and manage to live a long life anyways. There isn't anything that can be done about it though. If the bunny can't move her back legs you will need to be especially carefull that she doesn't get bunny poo all over her back end. If she does, you need to give her a gentle bath to get it off. Use a very mild dish detergent like Dove or Dawn and be sure to rinse all the soap off. Use slightly warm water and make sure you dry her as dry as can be. Make sure her cage is kept clean too. The poo can draw flies which is very serious as the flies will lay their eggs on the dirty fur and when those eggs hatch they will eat the bunnys skin. I know this is gross but you need to know this as well.

Maybe the best thing your friend can do is to try and find another home for the bunny. You might call your Human Society and see if they know of any bunny sanctuaries. There is a great web site
www.rabbit.org, which is the House Rabbit Society. Somewhere in the site they have a list of bunny havens and I think there is one in Pennsylvania.

I am proud of you and your friend that you care enough about the bunny to try and make it well. I wish you good luck. Think positive thoughts...bunnies are very sensitive and they will know if you are upset. Take care.