While I am male, I could probably be diagnosed with borderline anorexia. Speaking from experience, nothing really helps on the grander scheme of things. Being told that you are "thin" only makes you appreciate the person saying it, but not believe it to be true. The problem is that if you firmly believe that there is an issue with your weight, it's remarkably easy to dismiss everyone, up to and including professionals, as simply not understanding your concerns.
Whatever you do, don't say she has a "healthy" weight. That's just perceived as "you are not as skinny as society tells you that you should be and I'm not going to lie about it."
Really though, I would recommend just expressing non-physical things you appreciate about her when she's actively being upset, and complimenting her looks with delicacy and being non-insistent about it. Getting into an argument rarely makes anyone feel better, especially if you're arguing over your sincerity in a compliment.
Whatever you do, don't say she has a "healthy" weight. That's just perceived as "you are not as skinny as society tells you that you should be and I'm not going to lie about it."
Really though, I would recommend just expressing non-physical things you appreciate about her when she's actively being upset, and complimenting her looks with delicacy and being non-insistent about it. Getting into an argument rarely makes anyone feel better, especially if you're arguing over your sincerity in a compliment.