I don't know much about gastric bypass, but if it would still recuire for her to only eat really small portions, there might be a chance that they'll allow her to pay the child price for the meal. You can probably call and ask about it, but you'll probably have to bring some sort of proof for it.
But how you pay doesn't matter as it's really easy for the server to split your check. When I worked there I sometimes had guests who handed me like 10 different disney gift cards and told me to use what was left on them