I am going to play devils advocate on this purely just for balance as I imagine you have got a mega bill from the owner and think its unreasonable. In my opinion and if it was me I would get myself a 2nd hand door and have the quarter touched up. That said most people will want the full door either replacing and matching or filling and matching. Then they will want the quarter filling and re-spraying which will be a huge area to achieve a good blend. Couple this with the fact that they don't want to allow anyone but the main dealer to do this at around 90 an hour and I think you have yourself a big bill. I would imagine its gonna break down something like:
150 for the door could be a lot more
100 for the paint
360 for the labour
100 for the paint for the quarter
500 for the labour
All that could be done for probably around 200 quid by a dent doctor type guy but then if it was your car would you want to settle for that when it wasn't you that damaged it?
It a horrible thing and people will take the **** unfortunately.