whatever works... it is likely the cause of his being turned away at the airport ; I know several people in the States who has the same problem- I was flying once from Taiwan and the airline wouldnt sell me a one way ticket- I had to go to the Thai consulate and explain to them that I was headed for Cambodia by land and they wrote a special note to the airline for me.
have him buy any cheap air asia ticket out of Thailand for sometime before the visa expires- he can throw it away or use it - probably no return ticket is the reason
