Thanks everyone for your comments and advices - really appreciated.
Niece is now happily in Calif with her father. She said the flight was ok, she slept a few hours, watched 3 movies 
. The only thing that her seat did not work was the USB charger, but she could use her neighbour's. When the flight arrived at LAX, there were 6 other international flights arrived at the same time, so she was in the queue for customs for more than 2 hours! There were also queues for pax connecting flights, and queues for J and F pax. Other than the long wait times for customs, everything was good.
I have made notes to check seats at T-80 for her flight home, and to remind her to check in online 24 hours (and not to chance it at the airport check in counter).
