She is probably suffering from osteoarthritis of the knee joint. Get her radiological evaluation (X-ray both knees, AP & Lateral views) done. Start her on a course of physiotherapy with focus on posture management, precautions and knee exercises. For pain relief she can take a mild painkiller like Etoricoxib 90 mg twice daily. Depending upon the X-ray picture, intensity of pain and lifestyle impairments, a call to perform a TKA ( Total Knee Arthroplasty) needs to be taken.