When I was in a similar situation with Mr flyingbee unable to take the outbound sector on a normal non-redemption paid for booking for both of us (maybe 2 years ago, not recently). I called and explained the situation, but they said we would have to buy Mr flyingbee another ticket for the return. It was only ET and not too expensive, so we bought the new one-way ticket for the return.

Now, I did fly the outbound sector as planned. When we went to check-in for the return leg, we discovered that Mr flyingbee's return leg had not been cancelled, even though he had not flown the outbound, and the very confused check-in person said that he had two tickets for the same flight. We had bought another ticket and it wasn't needed.

Anyway, we went to the ticket desk and the kind agent there organised getting the taxes and fees refunded for the unneeded ticket, so we weren't out of pocket by too much.

One theory I have that the return leg ticket didn't get cancelled is because there were two of us on the booking, and one of us flew the outbound.

This was my experience, but I would never ever recommend that you could assume it would happen again in the future. I don't mind too much that we bought the new ticket that wasn't needed because at least we had peace of mind.