Your mother may be suffering from a condition called kinesiophobia (fear of movement). Chronic pain sufferers with kinesiophobia are so afraid that movement or physical activity will make their pain worse that they stop moving. Then they get into a cycle of movement avoidance, disuse, more pain, depression, and hypervigilance.
Patients with fear-avoidance behavior (FAB) of this type can respond to a specific type of rehab program. Usually a psychologist and a physical therapist work together to gradually progress patients through a cognitive, behavioral, and physical program.
Patients are asked to rate their expected pain before completing a task. They are guided through the physical activity. Then they re-rate the actual (experienced) pain. Usually, the expected pain is overpredicted. The actual pain is lower than anticipated.
Through this process, patients begin to see how their fear-based expectations set themselves up for failure. You are right that moving a little more would help. But she may need the supervision and assistance of a team of health care professionals to break free of long-standing fear-avoidance behaviors.