I would be mortified if I tried to travel with an expired passport. I would not be announcing to the world "How embarrasing for BA...." that travel was commenced on a domestic flight, with the passport just being used for identity confirmation by various airport security operatives. The system works- she was stopped from actually boarding to the States- where it counted. There was no embarrasment on the part of BA, just like there is no embarrasment to Tesco if you do a big shop and then discover you have left your wallet at home. I have used an expired passport as proof of identity at banks- as far a Newcastle was concerned, the lady had adequately proved her identity and had no need of a valid passport to allow her to travel to London. It really was her fault alone. To impugn the airline for this is going a little far. Sometimes people really have to be responsible for themselves.
my manner may be brusque. In the interests of brevity, one has to type different to how one says things. Face to face, it would have taken me 5 minutes to get around to pointing out what a daft thing to do and was it really anybody else's fault? But it would have been said nicely .