Osteoporosis is a disease in, which bones become fragile and more likely to break. If not prevented or if left untreated, osteoporosis can progress painlessly until a bone breaks. The T-score osteoporosis number is a number that indicates whether or not bone loss has occurred.
A T-score above - 1 is considered normal bone mass.
A T-score between - 2.5 and - 1 is low bone mass (called osteopaenia).
A T-score below - 2.5 is considered osteoporosis. You mother's T-score is 1.9 and she should be on some sort of treatment to prevent progression to full blown osteoporosis. She should take at least 1200 mg/day of Calcium and around 800 IU of vitamin D. Besides this she should start weight-bearing exercises.
The pain that your mother is experiencing is in my opinion unrelated to the Osteopaenia that she has. She might be suffering from early mild/moderate osteoarthritis of the knee along with some meniscal tears. To start I would get a standing knee x-rays of her knee and look for osteoarthritis (OA). If there is no sign of any OA then I would proceed to do an MRI of her knees and consult an Orthopaedic surgeon.