As a formerly morbidly obese person (see my success story if you like), I sympathise with your concern very much. I know my obesity as a teen drove my family crazy with fear for my future, but I mistook that as disapproval of me for being fat.
The thing is, and you know this, until SHE is fully committed to making the change, there is nothing anyone else can do. No exercise dvd, physical therapy, psychologist/psychiatrist, internal medicine MD, bariatric surgeon, or even concerned family, can help her one bit. Indeed, your efforts may well irritate her or make her feel worse about herself. It has to be her decision. Sometimes that's the hardest part!
I'd advise you to tell her you are backing off, that you love her and you stand ready to help and support her whenever she is ready to ask.
By the way, on my 45th birthday I weighed around 260 pounds (not my top weight, but where I had "settled" for several years). Over the next few years I lost about 150 pounds. The decision was mine alone, but I had a supportive family and friends for whom I'm so very grateful!
I wish great success to her! And blessings to her caring daughter.
In the end the love you take is equal to the love you make.