The fare rules of Thai Airways say, for partly used tickets, there will be no refund. What is a partly used ticket? If the ticket is booked for two people and one ticket is cancelled for the entire trip, is it possible to get refund for the canceled ticket, when the fare rule says no refund for partly used tickets?
"Partly used ticket" usually refers to a passenger not using a return flight or parts of a multi-leg journey.
If you have a booking for two passengers that's supposedly two tickets, so you should be able to get a refund for one of the tickets (assuming that's possible according to the fare conditions). I've never heard of that counting as a "partly used ticket".
In any case, to make sure just ask the airline.