I think the idea of maintenance is terrifying. I've set myself up for success by choosing a weightloss path that only includes things I'm willing to do for the rest of my life. Even though I know im losing in a healthly way I still fear that I'll regain. I've never seriously attempted weightloss before (I just turned 21) i would imagine if you have experienced failed weightlossnattempts it's very difficult to be optimistic.
You cant change how she thinks about herself or weightloss, you can only be a good example. I have found that my weightloss has spurred my mother, boyfriend, and best friends to jump aboard. I would shift the focus from skinny to healthy. If she makes comments that you are skinny enough or exercise enough or eat too little I would not engage and reply with comments about healthy choices.
"I'm just trying to be healthy"
"I enjoy being active"
"Workouts are really satisfying for me"
"I feel better when I eat healthier"
It's hard to be supportive when friends are a little toxic to your goals. I have a friend who feels that I must hate my body to be putting effort into changing it. That's just not true, I love my body, my curves, and my stretch marka, but I'm making the change because I'm young and I want to be fit.
Best of luck with this situation. Even if it doesn't seem you are being super helpful, just having a friend who she can talk to does more than you probably realize.