he/she is lucky not being sent back from the airport.
If you are a canadian citizen and going back to china. Get a two way ticket. chances are, you might be sent back to buy a two way ticket after you get to the airport.
It depends on airline and is just a requirement by the airline.
If you are rejected entry by a foreign country for any reason, the airline must take you back. To avoid the situation where you can't afford the return ticket, you were asked to buy it before hand.