But seeing as the Pax would stay airside during transit, didn't see the need to transit UK.
I still contend the first thing to do was to call the airline (BA?). Their reservations staff would have the same information (source) as check-in staff, meaning a fairly good chance of a consistent handling (although if it was BA, one never knows). Other information sources such as IATA are useless if the airline uses something else.
Good luck to the passenger.