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.
"No matter how developed you are in any other area of your life, no matter what you say you believe, no matter how sophisticated or enlightened you think you are, how you eat tells all." - Geneen Roth