I've had to have similar talks with my mother. I think it's important to tell them this: "You are the adult, he is the child. If you tell him it's time to go to the potty, you take him there. You are the adult."
Can you buy her an egg timer and tell her to set it for every hour or however often works for you? Grandparents can be a bit lazy or not realize how much time is passing. A timer might help.
I have to also remind my mother that she's not asking DD. We don't say, "Do you need to go to the potty?" because it's opening the door for a fight by giving her the option. We say, "Ok, it's time to try to go potty!!happy voice!!!" When she's gotten it down better we can ask instead of tell.