This distinguishes her from John's office manager, Jean. Back when she was talking behind his back about how horrible it was that he was stepping out on Elly with an ugly, ugly woman, she knew the risks inherent involved in talking about something when you don't have all the facts at hand. The end result of that little incident is that Jean and her friends look like chumps because they never found out that Fiona wasn't John's mistress, she was some distant relative he was told to make distant. This leaves us staring down the barrel of a rather awkward incident occasioned by Ms Brass's death due to her not really taking very good care of herself. It could well be that if the strip had been allowed to continue in real time, we'd have to contend with John telling Jean that the reason he's not coming into work that day is that his cousin Fiona passed away. We'd thus end up with a letter that had John opine "CHEE!! I just learned that back when April was a new-born, Jean and the gals thought I was having an affair with Fiona. Boy, was Elly sure mad!! You'd've thought she still had hormones!"
What John would have taken away is that women cannot be trusted to keep things straight in their minds ever. This is owing to both his letting stereotypes substitute for knowledge and to his not noticing a key detail. Jean remembers things well but did not have all the facts while Elly can't remember things clearly.