Upvote:4
"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.