My wife's boss died a few months ago, at age 56, of a blood clot that migrated from his legs to his heart and caused a massive heart attack. He was in airplanes every week, flying all over the place.
In the wake of his death, we learned that frequent flyers, particularly flyers who are middle aged or older or who have circulation issues, would be well advised to wear a special kind of socks that help prevent blood from pooling in the lower extremities, as this is apparently one cause of blood clots there.
Wonder if the OP's mother might give this some consideration.