Hi, dont worry to much if you don't see her for a few days. Checking the litter box and food are good ideas to make sure she is still moving around down there. She will probably come out on her own when she gets more comfortable. We have one we used to call the closet kitty, that is where she hid for the first 5 months when she came here. She is lots better now. The incision should be fine if she is staying dry and warm, and is eating and drinking ok. If all of this does not work then call the shelter and see if someone there can give you some help. Most of the people that work at them are helpful and they are going to want both the cat and your mom happy. You might have your mom just sit quietly instead of wandering around down there, she might be scaring the cat.
Vet. tech and pet owner for longer than I want to think about