I doubt if he had his truck insured for 200k. I'm sure the other drivers insurance company, can supoena his insurance company and find out what value your brother in law told his own insurance company, i.e. what he thought the truck was worth. He had to have insurance on it and I'm sure they could get that figure.
If he has paperwork on the recent inframe, he might be able to get that $ added on.
He could also probably get "rental" rate or loss of revenue out of them for while his truck is unusable.
I see guys trying to sell trucks at a certain price, and then want the value of the recent inframe added on, but I think they rarely get that $ back out of the vehicle. If I'm buying a used truck, yes, I care that it had a recent inframe, but it doesn't up the value of the truck by the cost of the inframe. I always figure if you inframe a motor, and spend big $ doing it, you better drive that value out of it, because no one is going to pay you that value. If its that recent, he should have the paperwork on it, and might have a case for that $.
The sad truth is, if he wants top dollar for his truck, (and it sounds like he does), he's probably going to have to pay a lawyer to try to get it. The insurance companies have their own lawyers and pay them everyday, and have a lot deeper pockets than I personally do. He may spend a bunch of money on a lawyer, and not come out any further ahead, and have to pay his own lawyer on top of that. Its not fair and its not right, but that's the truth.
If he can make the case based on what he's spent on the truck, plus what he gave for it, and show what he has insured it for himself/ or has a bank valuation on it for a loan etc.: I'm afraid just saying "I want $100,000", isn't going to cut it.
From a New Yorker cartoon: