fk: What do you think might have happened if either of them refused to fly the return leg
Thats easy, the return leg would have been cancelled or delayed until a replacement crew, (sense would determine both parties returned non-flying to base as they were both reporting each other), were sent out to recover the flight. No different in airline response to a crew member going ill down route.
Capt and F/O then interviewed at base, and a slim chance that one or both of them
may have kept their job.