Was with my grandma the night she died. Dad went off on trip. Still feel guilty, what can I do?

Normal feeling. Please reassure your dad that his feeling is normal and there is no need to fight it. May be he should pray for your grandma's happiness in her afterlife, be thankful for all she did for him, consider donating to her favorite charity or engage in some charitable activity that was important to grandma. If your grandpa is still alive, your dad should make peace with him, before it is too late.