I just happened to see this. I had a similar situation and didn't even realize it could be an issue. I flew other person didn't, and it didn't effect my return. I had checked in but we never checked the other person in nor did we cancel the ticket.

I still had the option to check the other person in for return flight.

When gate agent scanned my boarding pass on outgoing flight, a weird thing happened that she asked if I needed "extra assistance" as that came up when she scanned. I don't know if that was related or she misread the way my boarding pass scanned bc I'm an elite member? Not sure as that never happened before