I don't take any notice of vets saying a cat is nice young a healthy, what does that even mean? What did he do to confirm this? Look at her? He isn't psychic, saying she would be 'fine' how on earth does he know that?
say she needs another C-section? Say kitten or mum dies due to the birth, don't think any vet should ever give this sort of information!!
For me that is just a no, I have neutered cats that have just had a bad birth, a lot would have had another litter, may have even been fine, but I didn't want to put them through it again!! Certainly wouldn't with a C-section, just my thoughts.