It was my understanding that ordinary compression stockings are note effective.

However, please read this research report which used Class I compression stockings (whatever that is, but I'm assuming of higher than commercially available compression).

Recommendations

1. Your friend should talk to his/her doctor, and have the doc order/prescribe appropriate compression stockings or other approaches, including medication if warranted.
2. Keep your fee elevated as much as possible while sitting.
3. WALK to NRT!
4. Drink lots of fluids.
5. Additionally, while seated, try to do regular foot and leg exercises.
6. Oh, and be sure to do the same on the return flight.