I've encountered this on many occasions with past and present fosters.
Jersey has been eating pedigree her whole life, mind you she's only 2 (almost) but her coat is beautiful and with a beautiful shine, her teeth look great and doesn't stink considering she's on kibble. I also had a foster that was 11 and he was also on crap food his whole life and it was the same thing. He was in great shape with nice clean teeth and everything. He was adopted out and is still going strong.
I would never personally feed my own dogs that, but sometimes I feel I should just leave whatever foster I have on the food if they're doing so well.