I'm sorry to hear this. She must be unhappy or stressed about something. Has anything changed? Maybe she's upset because she can't go outside any more. She could be unhappy with her litter box or the litter that's being used. Most cats prefer uncovered litter boxes and unscented litter.

I've been using feliway plugins, added more litter boxes,and now I use unscented litter after using cat attract litter for a while. Your mom also needs to be sure to wash her bedding very well. I'd add some enzyme cleaner to the wash to make sure that all of urine odor comes out. I hope that she'll be able to find a solution for this problem. Good luck.