The Passenger should be advised

BA will pre-seat the customer subject to availability & seat requests
are never guaranteed and always subject to operational needs

Subsequent seat change

Should an aircraft or configuration change generate an automated
operational seat change that separates the party, the ‘request seat’
process must be initiated again. The travel agent would need to
remove and re-enter the TOGT keyword.

Members of the Executive Club and other oneworld frequent flyers are able to
reserve seats at different stages, depending on their tier.

Tier Seat selection

Gold/oneworld Emerald Free seat selection from the time of booking
for everyone in your booking and friends and family on other bookings,
who are travelling with you in the same travel class and on the same flight*.

Silver/oneworld Sapphire Free seat selection from the time of booking
for everyone in your booking*.

Bronze/oneworld Ruby Free seat selection from seven days before
departurefor everyone in your booking*.

*You must be travelling on the booking (or in the case of Gold Members,
one of the bookings) for these benefits to apply. They apply to British
Airways flights only and are not available on Basic (HBO / hand baggage only)
fares or group rate bookings. Exit row seats are not available to Silver or
Bronze Members on long haul flights until 24 hours before departure.
On some occasions, exit rows seats on short haul flights may not be
available in advance due to operational reasons.