Flight crew are not entitled to a first class rest seat (and definitely not out of Dallas). Furthermore the flight crew rest seats are blocked out in the booking engine by the commercial department. The J cabin overbooking profile also takes into account the blocked seat(s). Something smells extremely fishy about this incident.
I'm particularly interested in what stats you can bring up on the matter, by PM if necessary. In my years in BA I've never encountered a situation such as that, but I have encountered plenty of situations where outstations are carrying out unauthorised upgrades of friends, acquaintances or even complete strangers, occasionally with a suspicion of backhanders being involved.