Easiest way to argue that he did take the flight is through cell phone records. I assume he has a phone ... the carrier should be able to confirm the phone went off their system at (date, time) when he put it on airplane mode, and that it got picked up roaming at destination at (date, time). Having his phone connections correspond to the actual flight departure and arrival times still isn't concrete proof h did the flight, but it's most likely good enough for small claims court.

You should be able to get the cellular date from the phone company, though there will be a fee (claim it in court!). AC's actual departure and arrival time can be found on Flight Aware or any other tracking site. It's crude, but again, likely good enough for small claims court.