I would recommend your mother schedule an appointment with her doctor prior to continuing self-treatment. Her doctor may order blood tests, and/or x-rays of the knee joint. Your mother should be evaluated for osteoarthritis; common risk factors are age over 50, and female gender.
Your mother should discuss physical or occupational therapy with her doctor. It might also be helpful for you mother to start an exercise program, to improve function of the knee. She should talk with her doctor prior to starting any new exercise program.
For over the counter pain management options, your mother can try taking Tylenol, or anti-inflammatory medications such as Ibuprofen or Naproxen. She should follow the directions carefully, and speak with her physician prior to taking long-term. There are products that can be applied directly to the skin such as capsacin topical or aspercreme. There are also prescription strength products that can be applied directly to the skin to reduce pain. Your mother should visit with her doctor to a) determine the cause of the knee pain and b) to review which medication would be right for her to reduce pain.