I assume he didn't have an appraisal before hand? If not I would suggest he have one done, it will cost a few hundred but is well worth it. The insurance company will do the same. NADA has these cars listed correctly, not as a plain Regal so that helps.
He should gather any reciepts and photos he has and try to find comparable vehicles to his as far as condition, options, etc.
I am going thru the same thing since mine was stolen.
It has been 4 months and I am waiting now for a arbitration hearing then I will have a check.
I could have taken their check but decided there was a lot more room based on my appraisal and I would fight for it. My appraiser thinks we can get at least $3k more.
I was following this hoping there would be a good ending but looks like it is time to pick up the pieces.
Good luck and pm me if you have any questions.