No offense, I'd be embarrassed to even ask a chef or manager if I could pay less at a buffet simply because I intend to eat less than most adults, even if I am medically unable to eat more. The intent of a kids' price is not because a kid will necessarily eat less than an adult (as a kid, I could stow away twice as much as I do now). It's because many families, with one parent or two, might not be able to afford to go if they had to pay full pop for every child i their family. When my family used to go to Anaheim, my folks brought a cooler full of food and we'd go out in the car to eat it, because they couldn't afford a buffet for themselves and 5 children. They couldn't afford counter-service for 5 children.
I could maybe understand if an adult person in your party was unable to eat anything at all-if, say, the operation forced them to only partake in an all-liquid diet, but they wanted to hang out with you while you ate. But if he/she can eat anything at the buffet, and they're an adult, then they should expect to pay an adult price. Techincally, you could expand that logic to include supermodels (their bodies are the way they earn a living, to eat a buffet might mean to lose work). I could ask to pay kids' prices at a movie because I'm extremely unintelligent and wouldn't appreciate it on an adult level. I could ask for a senior citizens' discount at Petsmart, not because I'm 70, but my dog is.
To cushion the blow, have her take extremely small amounts of lots of food, so she can at least sample many things, even if only a bite or two.