Will,
It doesn't matter if he was an unwitting player; if they can show OoA was working with either illegitimate or proscribed parties, his trophies could be seized.

To extend your analogy further, no, you won't be prosecuted for buying a stolen car from that dealer, but the car will be seized and you will have to obtain relief from the car dealer (somehow).

Mick,
If your friend was unaware of illegitimate hunting operations, I think he will be 'off the hook' criminally, but still may lose his trophies (I suggest he NOT have them shipped just yet). If OoA was working with proscribed parties, he will probably NOT be 'off the hook' (because it will be the government's position that it was up to him to ensure that he was not dealing with such parties).

Wish him luck.

George