If she's signed an agreement to repay the loan, then she's legally bound to pay it. What she could do is to make a reasonable offer of payment. If the company refuse and subsequently take her to court, then she could make the same offer and will most likely have to submit a statement of means. She is unlikely to go to prison or to have the scooter repossessed if this is her only 'offence'. I'm no lawyer, though, and would advise you both to take legal advice on this, possibly via CAB. I do know, however, that the company aren't allowed to harrass her, so if she's getting difficult phone calls and/or personal callers, then you can take action.
The fact that she paid well over the odds for the scooter, well, nothing can be done about that now. I do agree, however, that there are companies who will do this simply because they have a captive market. All I can say is next time, shop around and don't give companies like this the satisfaction. It may be worth you contacting your local paper, radio and/or television and getting their consumer people onto it - local rags are often really good at this sort of thing.
Puddicat, I do appreciate your annoyance at them in crowded areas, and I will own there are people who have them simply because they're too lazy to walk. I'm pleased, too, that your mum appears well enough not to need one. She's a lucky woman. However, do be aware that some people need them. My own mum is 76 and she has one. She doesn't go into town with it, but she does like to trundle around the local streets, meet the neighbours and see what's going on in the world. She's widowed and she can't walk - hasn't walked since 2002. She can't even stand up straight and gets around the house in a wheelchair. If it wasn't for her scooter, she wouldn't get out of the house at all.