As he didn't get a BP, have him try and remember urgently the checkin desk he went to as presumably they could look at the sequence data and see who was handled prior to the latestcheckin time and work backwards. Maybe there should be some form of "check in " token you can get if there's a queue to prove you were there, with maybe it showing the time printed out and they refuse to accept it if abused (i.e. clearly you check in for 0800 flight five minutes late, presenting a token from 0500 when you went to the bar).