Cant be convicted unless he had knowledge that they were in fact illegally obtained by the seller. Which even if he did know that he obviously wouldn't admit that to the police and the police would have a hard time proving it anyways which is why its so rare that people get charged with it.
your coworker/friend is safe.
<-- Law Enforcement Major
see school does make you smarter after all
EDIT as OmNomNom stated if he was approached by the true original owner and refused to return them, then he has another situation in which usually results in the court forcing him to give them up and him not getting his money back.