What you're saying definitely makes sense to me, but I'm not sold on him falling into the category of trying to screw an employer. I asked my wife, an attorney, and she's flat-out unsure what the courts would say, given the facts Warwickben provided.
Nonetheless, I don't think it would hurt to at least consult the EEOC to see what they say. If they confirm he doesn't have a leg to stand on because he was employed by the temp agency, I'm sure they'd be as quick to dismiss him as you seem to expect. In that case, since it doesn't go any further, I'm sure he wouldn't come across to prospective employers in the future as one whose litigious.
BTW: I'm curious what you do for a living. PM me if you don't mind sharing but want it private.