Did she inspect the car when she collected it? She doesn't have to remember a bump, it isn't very likely but it could have happened when the car was parked. A costly lesson re the ins and outs of hiring a car I am afraid.

Never ever trust car hire companies. I had similar problem at Luton airport. I always check every inch of the bodywork and 99% of time I find few additional scratches. Once I returned car in the dark and to my surprise a bloke came straight to me and pointed out tiny dent on the roof! That will be 200 quid for repair and they promptly charged it to my CC. I knew it wasnt me as I drove from the airport to my mums house and parked it on the drive. I contacted CC company and disputed the 200. Never paid it, even thou car hire co threatened me on many occasions. I suggest you contact your credit card co. Always check the roof of a car.