I've flown reasonably comfortably (with an extender) at close to 400 lbs (and 6'-2"). Ask for an extender if she can't get the belt locked, as stated above they won't bat an eye, and there's a good chance they've got one in their apron anticipating her need. If she's pear shaped, it might be more uncomfortable for her as she may not fit in between the ...
I spoke with BA over twitter.
The code would relate to the rules and restrictions on the ticket. If
booking via our website, these would be advised at the time of
booking. Nearly all flights would have underlying fare types such as
this, and as you point out, 'A' fares would usually be restricted
First class fares. These would normally have fees ...
As fare codes, after the first letter, are airline specific and BA does not share the full list with the general public, you may never find the full explanation. You worked out much already by working out you need to stay a Saturday night.
It is a long and complicated code but those are not unlikely, even though they are rare.