I work as an assistant coach and our head coach addresses this issue fairly regularly in a way that I really like so maybe it can help you out too. When we have a girl (or a guy) who is a little heavier than what their frame should hold we always address it 1:1 in a positive and definitive way as part of a larger training model. In the case of your runner, we might pull her aside after a meet and say something along the lines of:
"I'm really excited about the way you're handling adjusting to college running. Its clear to me that you've been working hard and that's the kind of athlete we like to work with here at _________. Now that you've kind of settled into our training program, we really want to focus on consistency outside of practice. Things like getting enough good sleep, following our nutrition plan, and making it to the trainer after practice are going to make all the difference. Myself and the other coaches get that you're in college and and there are going to be days where you can't put it together, but its important that you really try to maximize your good decisions and minimize the bad ones. If you want to be a factor a conferences we need to get you stronger, more balanced, and at a better racing weight. The decisions you make outside of practice play a huge role in your success and we as coaches want to make sure you're taking that seriously." Then we'd talk about schoolwork for a while because freshmen are a nightmare when it comes to schoolwork.
Even though we barely mention weight, the kid will almost always say something along the lines of "yeah i'm hoping to drop down to xx lbs by the end of the season, I'll pay attention to that," and the problem is solved. These kind of kids know they're heavier than the other runners, and when we treat it like any other factor in running fast most are quite receptive.
Just my 2 cents though, hope it helps