Your question seems to have caused a discussion about travel insurance, I'm not sure that's what you had in mind. What do you want to know ?
As far as I know the party unable to travel will be marked as a No show but there will be no refund from Cunard. The other party can travel as a single occupant of the booked cabin.
(This is not from personal experience just from my reading of these boards)
It's being reported on a Live from on the Viking board that one of their ships is still docked in Cape Town so Queen Victoria is still waiting for her parking spot.