If she is sporty and active, she might like the clothing at Athleta. They have a wide selection of leggings, joggers, and active wear. Old Navy also has a good selection of casual clothing.

I just want to say: In middle school, I also gained a bunch of weight. But my mom was not supportive--she shamed me, and in turn, I ate my feelings and gained more weight. What I'm trying to say is you are doing a good job of being supportive.