Hmmm, stories like these are the reason I never rent cars with full LDW with 0,00 excess.
The few times I tried (mostly due to corporate agreements) were tiring with spending weeks of disputing some obscure damages.
Anyway, for you're case first thing I would ask them is to produce the workshop invoice showing what they did to the vehicle. If it was out for service 3 days they will need to show why repairing a tiny dent would take three days.