It wasn't AC (BA) but after being pre-warned they were actually very very good out of LHR even in Y. My friend was actually taken on board first and sat in F until everyone else had boarded then then led back to his seat.

AC should be able to at least come close to this level of service with some advance warning.

I mean, she's in J internationally so will have partial concierge service right, so AC should be able to take care of this. Make sure you Mom has a cell phone that works in the UK as well. I'd actually ask AC for the concierge numbers as she's in J, you never know.