I agree that any weight loss plan would probably be a good idea at this point. Even a healthy person has less stress on joints when they lose weight, and if hers are already aching...

Atkins would be a healthier alternative then having her try to force down slimfast shakes, and she may prefer it to WW.

If she is concerned about animal fats and doesn't really want to risk it even after reading about safety, she can still do Atkins without consuming many. Olive oil and other oils are excelent sources of fat. As are avacados, nuts and nut butters, cheeses and so on.

She should probably consult her doctor on what exercises she can do. I know many with arthritis love to walk or swim because it loosens things up. Other's just can't because of the pain. Water arobics classes also have the advantage of they don't place stress on the joints like dry exercising would (water exercises also burn a higher percentage of body fat then dry according to the YMCA)