Ummm, I think Grandma's big 80th deserves to come first. A coach that wouldn't respect a milestone family event like that, and the significance of a boy paying his respects to his grandma by being present... well what can I say--I think there are exceptions to certain "rules" and this would be one.
Let parents know that you must have 4 months advanced notice of any potential absences and you will make a determination, case by case, of those that you will accept as "excused". Those that are unexcused will result in sitting the bench for a certain number of games or innings. I think this family has given you plenty of notice to make an evaluation and plan around this special, once-in-a-lifetime event.