And that's assuming there's no third-party GPL code used in his project, because that would make the whole thing GPL regardless of who did it or who it belongs to.
So, I think the first two questions the OP need to answer are:
1) Did he use any third party GPL code in his project?
2) Was the project done 'for hire', and if it was, was the contract free of any licensing restrictions? (e.g. the company didn't ask for ownership of the code produced)
If the answer to any of the above questions is "yes", then he has a case, and he should seriously consider getting a lawyer (and/or the EFF?) to take a look at it.
If the answer to both is "no", then he'd be hard pressed to prove the GPL license was legitimate to begin with, but that will depend on the fine print of local laws I'm not familiar with.
Honestly, in any case he should seek advice from a lawyer, but answering those questions would give a better idea of how solid his case might be.
(of course, this is just an opinion, IANAL)