As a matter of interest if somebody turns up at the airport with a letter purporting to grant the holder some form of waiver / exemption, how do airport staff know:

1. that it is genuine, and
2. that the person who signed the letter has the power to issue such an exemption / waiver

I would have thought that if you had such a letter you would want to get it cleared with the airline (or both airlines in this case) before turnng up at the airport.