I am also interested in the answer if someone has practical experience. I had an almost identical situation - companion ticket booked around the holidays, then I had to travel for work on the same day as the return flight. My ticket was the paid ticket, and my companion's was the free ticket. The Diamond Desk advised that I couldn't skip my flight, that my companion would not be allowed to fly. By a strict reading of the T&Cs I can see how they got there. I somewhat doubted they'd come on the plane to unload my companion right before closing the boarding door, but I didn't test that theory.