It wouldn't matter, when he got onto the train to go home he would see he had the wrong ticket. IF he had been given 2 singles instead of a return, the only reason would be that because one single would be for a peak train and one for an off-peak one.
In such a case, the ticket would clearly state on it that it was an off-peak single bough during a peak time on the outward train and hence validate his story - but he didn't, he just swore a bit.
Either way, I agree with you, couldn't really care less either way.