Guessing it's more like the husband was not initially travelling, and when mother got ill, decided to bring husband along instead.

Wife phones BA to have mother's seat transferred to husband, is told ticket can't be transferred to another pax, hence misunderstanding "Was allegedly told that mother’s seat could not be resold"

In anger of now having to pay "extra" for husband, doesn't hear / listen to explanation of how mother's ticket is now cancelled. Having originally paid for seat selection, continues to consider the chair her own.

Then turns up at airport expecting that mother's seat is still empty and expects to be able to use it. Is shocked that BA have filled seat that she feels she's paid for.

At least, that's how I'm screenwriting it...