You may be a sore spot for her. She may feel resentment that you were able to lose weight. That doesn't make her a bad person, it's just where she's at. And it's not personal. I would suggest that you try to stay away from her. She may not want you constantly reminding her that she CAN do it, you may very well make her uncomfortable. Don't worry about her so much, just do you. Be positive, offer encouragement if she asks you for it but generally avoid her.
"If you pay attention to when you are hungry, what your body wants, what you are eating, when you've had enough, you end the obsession because obsession and awareness cannot coexist." - Geneen Roth