A.V.
Sounds like setting her up for an unhealthy relationship with food is part of what is going on. If meal time is a stress time, then it stands to reason that she has tummy problems. If my DD doesn't eat well, she doesn't get dessert (when we have it) and if she sees someone else eating, then maybe she'll be more motivated later. But it sounds like the father has weird food habits/weird ideas about food. If he is taking food from his daughter and basically harassing her all meal, then he's making her feel insecure about something very basic - eating. High achievers can also be perfectionists and if food is something the parents see as something to "perfect" it can lead to issues.
It can be very hard when you see behaviors that are harmful to a child you love, but do not parent. I would encourage the child to eat appropriately when she is with you. Talk to her about moderation but being able to enjoy even sometimes foods some of the time. Maybe get her cooking with you so she learns a different way to associate with food. You will likely get resistance from your daughter if she has problems of her own. You may need to encourage the granddaughter to talk to her pediatrician or school counselor.