I have taken care of many older adults after hip surgery, and swelling in the leg is quite common after this procedure. You do not mention if the swelling is
in the lower leg or the surgical site itself. You also did not mention if her other leg is swollen too. This data can help point to the cause of the swelling so that a remedy can be found.
If the swelling is in the lower leg(s) only, use of compression stockings can help. Also, the leg should be elevated when your mother is sitting in a chair to prevent fluid from collecting in her leg. The most common cause of this type of swelling is venous insufficiency, which is very common in older people.
If the swelling is in the surgical area, I would keep an eye on it. It is quite common for some people to get swollen in the surgical site, just due to the surgery itself. If the area becomes inflammed, painful, red, or warm, call her health care provider immediately. This can be a sign of infection or a clot.
One of the things that will reduce swelling after surgery is physical therapy, which encourages gentle movement of the limbs (which will help the fluid move out of the tissues, and back into the circulating blood where it belongs). Please encourage your mother to participate in her rehab as much as she can. Good luck.