I recommend a second opinion. 80% chance it will help? Those are pretty good odds IMO.

13 too old for a dental? Well of course it depends on the cat, by my senior girlie was 17, had CRF, hyperthyroid disease, megacolon, arthritis, high blood pressure, heart murmur and one previous stroke, and she did just find with a dental and an extraction.

(My vet uses Sevourflorane gas anesthesia.) It gave her a new lease on life, as her teeth had been troubling her more and more while my vet and I debated the risks. I finally decided quality of life was more important, and if the anesthesia hastened her end by adversely affecting the kidneys, at least that shorter time with me would be spent without mouth pain.

As it happened her kidneys were not affected at all by the dental and I had many more months with her, and she was so relieved to be free of mouth pain, and her appetite improved, which is so important.

Again, I would advise your mother to find another vet. Sometimes the risks outweigh the alternative, which is a lifetime of mouth pain and slow death by starvation because of it.