This is a tough one. I might go ahead and make another one, but I would pointedly tell the dad that out of the kindness of your heart you're doing this, because that cake was dropped and really not your responsibility but you don't want to have a disappointed grad.
Then I would never send a cake that wasn't cold to be driven for an hour in the trunk of a hot car (not sure if it was hot or not, just assuming). And then just to CYA some more, if a customer is picking up an important cake, take a piece of that rubber mesh drawer liner and put that underneath the cake so it will not slide in the car. I am also assuming you were the one to place the cake in the car? If not, always be that person.
Then, with signed paper in hand absolving you of further responsibility, wave goodbye and good luck because it's all in their hands now!
I agree, Make another "But be SURE to show the dad , The before and after,
This is what left my shop------This is what your daughter bought home