I feel it only fair to warn you right off the bat that the scenario I’m about to write probably wouldn’t happen in the real world and probably doesn’t need to. After all, all of us go through our lives never knowing that certain people think certain things about us that just aren’t the case so it doesn’t really matter if John never learns that his office wife thought that he was having an affair because he was going through a rough patch at home. This little glimpse into either the present day or near future is simply me scratching an intellectual itch.
That being said, I think that we can safely agree that even though Fiona is only about three or so years older than John, she’s lived a hard life and taken poor care of herself. We can thus expect her to, if not die outright, have the same sort of severe heart attack that Mike’s muse Agnes Dingle had a while back. This, I think, should lead to a rather interesting conversation between John and Jean. Since he hasn’t the vaguest idea that Jean has cast Fiona as the other woman, he’ll have no real idea why she tenses up when he talks about her or why Jean keeps asking him to explain what he means by saying that Fiona’s just his cousin. One “CHEE!! You act like I was having an affair with her” later and he’ll finally learn about the misapprehension Jean has been labouring under for twenty two years. He’ll also learn that Jean only pretends to respect him because she wants to keep her job as well as learning that Jean feels sorry for Elly having to be married to an unsympathetic buffoon who treats her so cavalierly that adultery would be par for the course.
Since he’s rather averse to seeing the sort of nitwit we see, he’s not going to take the revelation that he’s thought of as the sort of bastard whose response to a new child is to run off and chase something fluffy down a rabbit hole well. I also don't expect her comment about how the only time that he didn’t make his life an open book coming back to haunt him to sit well either. The only person I expect to be pleased by all of this is Elly. The idea that Jean has her back no matter what would certainly make her feel better than she has in years owing to her clear belief that no one is in her corner.