I doubt it would have been allowed simply because he was wearing it so it became part of his clothing. I suspect that the chances are he checked in online and it wasn't questioned by security or gate staff. Or it could be something a little more bizzarre...
In the mid noughties I was flying with an airline which issued handwritten paper tickets. The agent (employed by the airline) wrote a departure time that was an hour later than it should have been. Through no fault of my own I turned up after check in had closed and was eventually escorted through security with my 'checked' luggage which I was forced to carry onto the plane (an ex PIA 747 classic) as hand-luggage. Needless to say, the cabin crew were horrified, especially as the flight was full and I was the last to board!
(p.s. this happened at HKG; the airline was Orient Thai. Not sure on the exact year but I think I had shampoo so it must have been pre 2006...).