You can intervene without it being a big deal. Don't build it up in your mind that you can only say something in the form of some lecture or argument that changes the relationship. You only need a passing, directed comment about healthy eating and healthy body weight. If she's as intelligent as you think, you could probably even do it through metaphor or a story about a "friend" who just so happens to be overeating and gaining weight and should eat better and keep it really casual.