Has something else happened in her life that might be getting expressed as a weight and food issue?
Relationship changes, financial problems, things like that?
If there is something like that going on, I'd talk to her about the problem rather than about the weight/food.
If there isn't anything stressing her out, then could it simply be that she's making a concerted effort to get fit? (you said she looks really good)
When I was anorexic, it was totally a symptom of a different problem. I got a lot of flack about my eating and my fragile thinness, but no-one addressed the issue behind it... so the harping on my eating never connected and I never even realized that I'd had a food problem until years later.
If you are concerned, maybe ask her how the rest of her life is going? And leave the food alone for right now?
tl;dr - food issues are a symptom of something else gone awry in someone's life
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson