I would have thought that airline records would be considered absolute proof
I think the OP's point is that the airline are defying his client to provide independent evidence that he was on board the flight.
I'm not a lawyer, but I'd have thought the onus would be on the the airline to prove that he
wasn't on board.
But then again, if I was a lawyer, I'd have told my client he was an idiot for chucking away his boarding pass.
