There are several factors to consider here. First of all, is your mother getting better from the time she started until now? She should have less (or no pain) by now and increased function.
Sometimes after the acute episode of back pain is taken care of, a conditioning program is needed. This is especially true for older adults who may be too sedentary.
It might be helpful if you went with her to her next appointment. See what she's doing and ask some questions. It may be a good time to switch to a community exercise program. Is there a local YMCA or other similar type of health club that offers exercise programs to fit her needs?
Exercise may not be the only benefit your mother is getting. She may enjoy having a reason to get out and be with other people. Perhaps you could ask your mother what she thinks the program is still doing for her after this much time.