The orthopedic surgeon is really the best one to advise your mother and answer your questions. He or she has the benefit of knowing your mother's history and the results of the physical exam. Looking at the joint and leg alignment helps guide the decision. Seeing X-rays of the joint space is also very helpful.
At age 66 your mother is still fairly "young" by today's longevity standards. If she has severe enough joint damage, then total joint replacement may be the best option. But these days, the goal is to preserve the natural joint for as long as possible.
It sounds like she's planning to have an arthroscopic debridement. This is a minimally invasive operation. The surgeon makes two or three puncture holes and inserts a long, thin needle (the arthroscope) with a tiny TV camera on the end into the joint. This tool gives a view inside the joint. Tools used to remove loose cartilage or to repair any damaged cartilage are passed through the scope.
Most patients are up and going two or three days later. They wear a knee immobilizer and put partial weight on the leg until they feel up to full weight-bearing. Range of motion exercises are prescribed. Most pain relief occurs within the first six months. Some patients report continued improvement for up to two years after the operation.
It's a good treatment option for patients with mild osteoarthritis. The ease of recovery makes it worth a try before going to major surgery like a joint replacement.