I would say depending if the bumper is fibre glass or plastic.
if its fibre glass it can be filled smothed over and then resprayed if you go to a good body shop.
If its plastic, im not saying it will defenily work but my mate has an Astra (fool) but anyway he did the same to his, took the bumper off heated it up with a hairdryer until the plastic went softish and the dint pushed straight out. if it works then you only have paint to pay for.
But I would say if you think he was an honest guy and you think he was going to come and tell you what he did if you didnt go out then keep it as cheap for him as possible, but if he was about to drive away before you caught him them go the the most expensive place you can find and have his eyes out.