I dont give answers, I give arguments for either posiiton - that's what lawyers do.
The seller is stuck with a potentially siezable car. You wanted my opinion, that's my opinion. Whether he tells the world or not, does nothing to change the car's status, it only affects what level of liability any present owner faces from trying to pass it off to the next guy. It doesn't change the fact of the car's tainted heritage and the end-result it faces today based on that heritage.
My advice to him, as unplatable as it is, is this: Take it to the motor vehicle department/highway patrol, explain the status and try to get a state VIN issued for the car. I think that is the only antidote for the poisoned VIN status of this car.
We all know the value of one of these cars is intrinisically linked to the VIN code but this car's VIN-based value was destroyed years ago when it was rebodied.