Has she ever acknowledged the fact that she needs to lose weight? Is she disciplined in other areas of her life? Has she always been overweight?
She is likely very fearful. Doesn't know how or where to begin. Fears failing, fears change, fears the unknown.
There are a lot of hospitals out there now offering various classes - not over eaters anonymous, but similar type group classes. Weight Watchers is good for someone who's ready to change and needs a start. They teach good, basic skills for the severely overweight.
The first step is acknowledgement. She has got to WANT to change. After that, it's just a matter of finding the right fit or combination of resources. You and/or the gym are not the current answers.