If you are flying on a mid-J 747, then I think you are right about not asking her to wait in her seat. However, and I speak under correction here, disembarkation of mid-J 747s is usually only using one airbridge at the door behind WTP and in front of CW. So if you ask her to go through the airbridge and wait for you at the end of it, that would probably be the easiest thing.

(Ask at check-in for AUPs... You might decide in the heat of the moment that it's worth upgrading to WTP and sitting with your mum... )