Based on the information given by the OP, they had 9 people in 4 rooms. That means 3 rooms of 2 and one triple.
They maybe could have moved to granddaughter to the triple turning it into a quad but there are also lots of rooms on Harmony that only accommodate 3 so it is quite possible that moving the GD to another room was not even possible. This may have been the only option to preserve the rooms and in this case all they would get back is the taxes.
What should have happened and this would be the only refund they would be entitled to is the third guest and the daughter swap cabins putting the daughter who did not sail in as the third guest and then cancel her after that change at least getting a small amount back.
I would suggesting contacting yje executive offices and see about getting the refund gor the third guests fare.