Support stockings may help... but it depends what is causing the swelling. Presumably doctors have already checked... but basically, if you press your thumb into her swollen ankle, and it stays "dipped", the problem is excess water... and water tablets may well help if she isn't already taking them.
Keep her legs raised as much as possible, rotating ankles or exercising, if she is able to. I would personally find out the cause of the swelling first x