A customer complaint!!! What to do???

Ok so I had a customer have their 18 year old pick up their graduation cake today, party tomorrow, and when she left we told her how to handle the sheet cake and lay it on a flat place in the car. So an hour later the dad calls complaining how the cake looked like crap it was falling apart and the "frosting must not have been set up" those were his exact words. The cake had been done for 5 hours so it was "set up". Now my question is what to do about it? We are so booked this weekend with cakes that to do a whole new cake double layer sheet cake would even put us here that much later, especially when I don't think it was our fault. Whatever happened, obviously happen in the hour it took her to get home, I am thinking she may have put it in the back and turned corners sharp the cake could have been sliding side to side hitting the edges of the car? I don't know? All I know is that it was perfect when it left our shop.

So I told the dad to bring the cake back we could look at it see if its fixable or what may have happened but he acts like he wants a whole new cake.

This is not the first time customers have picked up a cake from my shop and just "throw it in the back seat" so to speak, I just don't understand them its like you spend so much on the cake and its the centerpiece but yet instead of putting in the back of the SUV on a flat surface they put the beer there, really??? Even after warnings of what may happen.

Sorry for the vent, but do you all use forms that they have to sign when they pick up the cake that everything looks ok and your not responsible after it leaves the shop?? If so can I have a copy of it lol

I don't have time to do another cake but I don't want this girls graduation party to be ruined either, what to do?????

yes I took a picture, the frosting is the same recipe I always use and never had a complaint, like I said it was sitting there for 5 hours before she came and got it, the only thing I can think of is she probably made some stops before home? The cake might have slide from side to side? I don't know, obviously I am not going to approach it this way to the customer but it just ticks me off when people don't use common sense when driving with a cake, yes it is only a sheet cake but still, you can't take corners at 90, not saying thats what happened, just saying...
I am getting forms for ever customer to sign from this point on...

I posted a before pic and after pic of the cake when she brought it back...what do you think happened? To me it looks like it was dropped. It wasn't a hard cake to do and I wouldn't mind redoing it if they would be honest about and if the dad wouldn't have called and talked to my employee the way he did. I understand accidents happen, but this cake sat there for 5 hours and was fine I don't see how it could have went from that to the way it was an hour later?

I would refrigerate it but we are a small bakery and our fridge isn't big enough to put every cake in there....like I said never had a problem like this before and the weather isn't any hotter than usual actually cooler today.