I don't know why you vet said you would have to wail to spay her. My cat was spayed as soon as her last kitten was weaned. The only reason to wait is so the nursing relationship is not disturbed.
(BTW, I was adopting the mom cat and her last kitten; she was turned in as a pregnant 'teen-aged' cat)
(BTW, I was adopting the mom cat and her last kitten; she was turned in as a pregnant 'teen-aged' cat)