Well, I can tell you what I did with this EXACT same situation -- although my parents aren't a-holes or bulldozers... so maybe not exactly.
I gave them a heads up that so and so was a picky eater, unlike me as a kid, who ate enough of most things so-as not to cause alarm about complete lack of vegetables. Did they want to know more?
Yes.
So then I told them. I said that we have tried to encourage so and so to eat more vegetables and novel things, but she refused, it was the source of great acrimony, and I did believe she truly hated them. And I could relate, because when I was pregnant, things that I used to love or at least didn't mind, actually repulsed me. As a pregnant woman, I had the eating inclinations of my 7 year old: white bread, mild cheese, citrus everything, ice cream any time. Green vegetables and anything with meaty or pungent flavors ore smells were O.U.T of the question.
And then I told them how I handled so and so's eating. It wasn't my first choice, because I would have loved to have hear eat veg, but it did satisfy another goal: avoiding nightly fights around the table. So if grandma was starting to have these fights, she might consider the strategy I landed on--or not. It was up to her.
But who knows? Maybe so and so would find G-mas cooking amazing and she'd try something new without me probing and pushing.
And then I let it be.
Later, in a Skype session, I saw my kids going to the grandparents' fridge to help themselves to chocolate pudding for breakfast. The kids reported no tantrums. Okay, so my kids were eating crap, but they were having fun, Gma and Gpa were doing me a big favor, and they weren't yelling at the kids... all that and, hell, it was just two weeks.