I'm not fluent enough in halacha to answer, but lets try to take our negious out of it for a minute.
Case 1) Someone comes to me, buys a ticket to a movie at a theater I don't own, I just sell tickets for on commission. He shows up and the show was cancelled. I don't want to refund the purchase. Any halachic issue to dispute?
Case 2) I sell the ticket, and the theater processes the payment. He shows up, and there is no show. He disputes the charge, and the theater comes after me. Any issue with him disputing it?
If someone buys something, and doesn't get it back, they are due their money back from whoever they paid it to. Everything else is between the TA and the airline.
The anomaly here is force majeure, which might release TA/Airline from responsibility, but other than that I don't see why the customer has to worry about what damage he's doing to others if the money is rightfully theirs