Keep in mind that many seats with the best legroom, such as 26ABHJ on the A300s, are also exit rows. If her condition requires extra legroom it probably also disqualifies her from exit row seating. If this is not mentioned beforehand, they'll notice it at the gate or on the aircraft and will reseat her then. At that point her choice of seats will be much more limited than it would have been earlier.
You might want to call the PLT desk and ask for a supervisor.