Hi. Your mother may have injured her ligament which can be diagnosed by examining her and confirmed on MRI. It would be better to consult with a pain doctor to avoid any damage.

Consult Spine and pain physician

Knee pain without any changes in X ray, can be because of ligament or miniscus etc injuries, which are not visible in X ray. Physical examination will help to diagnose the condition and for exact diagnosis MRI will help. But MRI is not needed in all cases. Whenever clinical diagnosis is not confirmed and to know detailed information MRI is helpful. Majority of these conditions can be treated by non operative methods. With good results.