You said she was overweight already right? You'll want low impact exercises or activities, and things that she can see improvement or have fun with, things easy on the knees and legs. It is hard to suggest without knowing her, but the first activity I enjoyed was yoga because the emphasis is on being gentle and cultivating flexibility. I also enjoyed stationary biking with friends, and lifting was a ton of fun once I had someone who could show me how to do it. She is at a great age to cultivate muscle and extra weight means she probably is stronger than she realizes.

Doing a morning stretch routine helped too. She wont be able to dive into something if she lacks confidence so developing a small activity that is easy to do to show she can make a change is crucial. Ither than that, tracking her steps and knowing how many calories in a day she needs to maintain versus lose, and taking a week where she just writes it all down without beating herself up about it and seeing the numbers is helpful.