Doh.
This was her seat.
In other words, it was the seat number on the boarding pass she likely printed hours or even days in advance, probably before EZY had determined which aircraft was going to operate the flight.
The lady was moved to a spare seat once the flight was fully boarded.
Far easier to identify an empty seat once everyone is on board.
Not sure what would have happened if the flight was full.
If the flight had been full, then clearly x passengers would not have been allowed to board an aircraft with x-1 seats.
One has to wonder how safe the rest of the plane was.
Oh dear ...