I guess I'm the only one who puts cakes in the back seat to drive them to places?
I do make sure it's level, though. At first, I used a combination of books and towels, and seat-belted the cake box in. (I ALWAYS get a larger box to travel in, even if it's a smaller cake, for safe transport) My husband has since made me a 'leveler' for the seats of both our cars. I have four... one each for the front and back of both cars, and they're labeled. As long as I use those, a center dowel, and a large box so that I can safety-belt it in, I haven't had any problems at all.
Oh, I guess I should also say that my trunk is NOT flat, in the slightest. Even if the cover is on the spare wheel well, it's still a little wonky, so I've never put a cake back there. It's also where all our spare fluids and cleaners are, leaks if it rains, and generally just isn't where I'd choose to put any of my food for longer than it takes to get it from the store to the house.
I'm very sorry that this happened to your cake, but it really does sound like driver error, and I agree that if the one who bought and transported it hasn't complained, then she probably knows that she's not due a refund.