If she had already gotten rid of her middle seat assignment at the kiosk, she would have had a new boarding pass... and no reason to talk to the people at the gate at all.
I suspect she's not telling us the full story here.
Good point. It may have been an honest mistake; she may have thought she'd changed it but somehow it didn't "save" the change or something, but... yeah, I bet she flamed out on the crew and the pilot wasn't having it.