I think BakingIrene might be on to something and the entire thing slid, not a collapse as the customer suggested. In my experience in driving a cake, which admittedly is sparse, I have been really surprised at how slow I have to drive and how the laws of physics affect a cake. I am aware of it now..but I'm pretty certain that a regular customer, while she may have known to be careful with turns and such, had no real understanding of how slow and careful you must be. I'm betting that she was watchful of traffic, but maybe took the turns a little too fast for the cake to survive. Even coming to a stop is difficult and while we might not even notice the forces at work, that cake will. That being said, it's not your fault that the cake failed after it was in her possession. If I have groceries in my car and had a problem where the milk jug tipped over or the eggs got smushed in the car ride home from the store, I can not hold the store responsible and expect any sort of refund. People have a tendacy to purposedly diminish their own responsibility if they think they can get someone else to take it for them. If she exaggerated about how far she was driving to meet you, she most likely has gone with the narrative that your cake collapased, not that she may have been careless and assume that the cake was not as delicate as they are.
Im sorry about this for you, the cake is beautiful.