Both short-term (two years) and long-term (four years) studies show better results with total hip replacement (THR) for older adults who are healthy and active. They have less pain and return to a higher level of function compared to patients who have the hip repaired with internal fixation devices (screws, pins, plates).
Many elderly patients who have internal fixation for hip fracture end up with so many problems, they eventually have the hip replaced anyway. By the time they have two or more operations, their walking ability and quality of life have declined quite a bit.
Decreased activity puts them at increased risk for even more health problems. And they are even at risk for new fractures of the lower extremities. So although it seems like more trouble than it's worth, hip replacement is advised over internal fixation for patients like your mother.