Knee pain, Overweight, Weak. Knee pain?

My mother has knee pain since last 2 years. She could barely walk. She consulted an Alopathy doctor. He asked her to reduce weight, as her weight then was 68. Since last 3 months, She started walking 6kms daily and eats only 1 chapati in dinner, only salad in lunch, a glass of milk at night, and 2 cups of tea in a day using 'sugar free'...Her weight has dropped down to 64 now. But still her knee pain didn't reduce, and She feels very weak throught the day... She had had uterus operation (removed it) due to fibroids 3 years back... She wants to decrease her weight further but feels weak if has diet as above... So please suggest me what to do...And how to cure her knee pain??? **she doesn't have any other joint pain