A long time ago we had someone on the forum whose mother had an undiagnosed spinal fracture that was causing incredible pain yet they were unable to express what the problem was - sometimes "irrational fears" have a real, rational explanation.
Why is she terrified of walking? You have no information on your profile, so there's nothing to go by! Does your mother have dementia? How old is she? Does she fall a lot and that's why she's afraid to walk? She may need physical therapy to show her that she CAN walk, and how to go about doing so properly, and/or with the aid of a walker, so she can feel secure and not petrified.
My mom, with aphasia from a stroke, was "resistant" to doing PT. She had an undiagnosed broken hip.