I absolutely misunderstood what was being said. lol I am sorry.
About her mom not thinking anything is wrong with her. I highly doubt that worms is making her favor one leg over the other. But about the worms to be safe, at Walmart they sell Over the Counter worming pills, for about 2-3 dollars. I would have her buy some and see if that helps.
About convincing her parents to take her to the vet, I dont know what to tell her to do. I wish they would do it because they want to, but if not, maybe she can contact a low cost shelter to see if they can check her. Or maybe if they just dont have the money, she can ask the vet about a payment plan. I know my vet only charges around 23.00-30.00 for a vet visit. worming is 7.00. So in all it isnt too expensive.(x rays in turn DO get expensive sometimes) She could have an internal infection that is causing her legs to get weak. Thats about all I know. Maybe other people could tell you what is wrong with her that have more experience. I worked with a vet for over a year, and I have never heard of worms causing the leg to hurt. But I could be wrong. Hope this helps.