Well, if the house was presented a gift from daddy and grandpa, that's who she should thank. Maybe in her note to grandpa, she could include the line "and tell Aunt Mary thank you for helping with the painting"?
Or use the logic that says, "Aunt Mary's help was given to FIL. So he thanks her."
Because you wouldn't thank the wholesaler who sold the plumbing supplies to your plumber, right?