How would you know he hadn't admitted to smoking a doobie or two in the past and that was enough to bar him from travelling?
But if it was "in the past" then he wouldn't have been accepted on any of the more recent previous occasions!
My money is in line with DaveR, most likely mis-identification. I believe that the only way to resolve that is indeed through the US Embassy. Meanwhile the poor guy is out of pocket through no fault of his own. Perhaps travel insurance companies should cover this eventuality in their policies (as long as it is proven that the claimant was wrongly denied boarding by Border authorities).