In the case of certain cancers, particularly ovarian and colon cancers, an abdominal tumor may get big enough to press on the main veins that bring fluid back to the
heart from the legs. The weight of the tumor causes the veins to narrow, so it takes more pressure to push the fluid through the veins in order to return it to the heart. This causes fluid to pool in the legs and feet.
Gravity is the main culprit in this situation, so you want to combat gravity by having your mother elevate her legs as high as is comfortable (the higher the better), as much as possible. Support hose are often recommended for this problem, but these can be uncomfortable when there's a lot of fluid, and many patients don't like them. Some patients like the feel of elastic compression bandages better, so these are worth a try.
Because the goal is to get fluid circulating back toward the heart, massage can also be very helpful for this type of swelling. You can massage your mother's feet and legs yourself with upward strokes, or you may want to find a massage therapist who's skilled in working with cancer patients.