My beef with this whole situation is that they punished her by trying to take away her home junior slam when she is the number player in the WORLD. I don't care that her recent results are below the USTA expectations. She earned that #1 ranking by winning the AO junior slam singles and doubles. It's hers until someone takes it away. So if a just turned 16 year old Lindsey Davenport or Martina Navratilova or Petra Kvitova, all who were known or are known for there perceived lack of fitness, played for the USTA PD they would have been kept out of the junior slam.
Girls' bodies are still developing at this age and I doubt that she isn't fit in the cardiovascular sense of being fit. You shouldn't force diets on teenage girls who are not obese. The developing brain of an impressionable teenage girl is sensitive. I know because I have one. You say, "Darling, you need to get into better shape" and they hear, "Hey fatty, you seriously need to lose that butt." And the fact that it's public, on the internet, the subject of tweets will only add to the humiliation. Continue to emphasize her fitness training but don't keep her out of competition. How does that help her?