Well I just went though the same thing. The bumper cover can either be fixed (less material cost but more labor) or replaced (more material cost, less labor). It will probably end up being near the same cost in the end for either.
With that out of the way the estimates I was getting to repair or replace the rear cover, paint and materials ranged from about $560 to $690. Even though the impact was VERY minor the impact beam underneath was bent also. That added another $250 in parts and labor. Total to fix was about $950.
Depending on his deductible it may be better for him to pay out of pocket. But expect to add the cost of the impact beam also just to be safe. The guy that hit us had expired insurance so I just went though my uninsured motorist policy. No deductible. The insurance company will go after the guy. I suggest you contact your company and GET A POLICE REPORT....