When flying and working (I work on my feet for long hours), I wear knee high support stockings with ~30mm of compression. I would suggest stockings with a similar level of compression for anyone on any flights over 90 minutes.

For long haul flights, I suggest wearing this type of stockings during the flight and for 12-24 hours after arrival (including while asleep).

From your description of your mother's problem, I would strongly suggest she be evaluated by her physician.