As far as I've been able to determine there is no law against using someone else's ticket. It would be a violation of the contract between the purchaser and the airline and possibly a civil matter, as long as the person who flew entered the gate area legally. The airlines like to call it "ticket fraud" but the airline does not actually lose anything of significant value, merely the opportunity to charge someone more. Some airlines have (or had) an option to change the names on already issued tickets. Back when I flew Airtran all the time we had an A2B account - one of the perks was to be able to change the name on a ticket at no cost. All it took was a phone call. We occasionally used that to substitute people flying for business . The airline could of course stop someone from boarding if they suspected that the name on the Boarding Pass didn't match the person holding the Boarding Pass, The most obvious obvious case would be a woman holding a boarding pass with an obviously male name, however many names aren't obviously one gender or the other and some trans people haven't legally changed their identification, I knew someone in that condition.