[Blueface]

Being in the fraud detection business involving staged accidents and damages and given my background in a body shop and as an adjuster and manager, I say very well done by all of you that have assessed this to be a backing up type impact as opposed to a sideswipe.
Clearly evident by the door not having a major impact to go along with what the fender sustained.
As far as your $500 assessment, consider the following:
Cost of Fender - probably in the $150 plus range
Cost of labor to remove old fender and install new one - probably around $45-50 per hour, minimum, more if at a dealer
Cost of materials to paint the new fender - very expensive nowadays
Cost of prepping, priming and painting the new fender
Cost of inner fender skirt - plastic and clearly damaged
Door OK or does that need to be addressed and blended for color matching?
Cost of clearing door to match once blended
Any damage to suspension? Any chance the wheel got whacked and now you will eat your tire?
Any damage to the rim? That alone can be from $250-$500 easily.

I would inspect his car closely. Impossible for him to not have sustained some resulting damage. Does he have a painted bumper or chrome?