This made me smile even harder. I can empathize with how happy the owner is. I had a very similar situation with my cat - vet gave me a 2% change it wasn’t mouth cancer. I spent 2 weeks in tears and then I came in to get the results and it was stomatitis - my cat’s stupid body was just allergic to his teeth and was rejecting them.
That’s basically how they explained it. His body was recognizing his teeth as foreign objects and was rejecting them. So his gums were all swollen and ulcerated. Got his teeth pulled out and he’s all better now.
Note - He still has his canines, they weren’t causing him a problem but I have to keep an eye on him and get those removed if they become an issue.
Can you try a different vet? I don’t know anything about cats, but I’ve got a senior dog who’s been happy with just one tooth for the past few years or longer (she was like this when the shelter got her). She eats regular food and everything.
A lot of times it gets so bad that the immune system actually starts eating away at the teeth(!)
You can treat the infection with aggressive antibiotics and clean the teeth, but it’s likely to return unless the owners brush them every day and very few people are up for that (and it might just comes back anyway even if you do).
So best practice is usually just to take the infected tissue and teeth out, as this actually less painful for them both immediately and in the long run.
It’s more a problem with cats than dogs, as you can give dogs dental cleaning toys to chew and it’s much easier to brush their teeth.
If anything, it’s a result of kitties not having bones to chew like in their natural diet (do not give your cat bones! Bones can splinter and cut up the digestive tract, cause an obstruction, or fracture teeth.) Making sure your cat eats at least some dry food can help, though.