you need proof of intention of leaving thailand. It does not have to be a flight. Not everybody travels on return flight tickets travelling around various countries. Airlines only have to protect themselves that travellers are moving onto somewhere else with proof by any means. Doubt an airline after discussion would refuse you once you produce an itinery and receipts of travel out.
is this a new process having to cancel your extension of stay before leaving the country with no re-entry permit? Or is it because he renewed his passport and is leaving before transferring the stamps?