I think it's not OK that they used this babysitting time that was allocated for a medical procedure, to go shopping, especially since they didn't * ask * but rather just *did*
However, I was wondering, how handicapped is you grandpa? How much does this make your grandma housebound, so to speak? Who usually helps her out?Because, while it wasn't OK,, and it certainly isn't the OP's responsibility, I could see her thinking in terms of 'yay! Thanks to granddaughter, I have a few hours of 'freedom''