I would look into doing olympic-style moves - like clean and jerk. This will help strengthen entire upper body, plus it is explosive, which can help in V-Ball.
Along that vein, plyometrics would be great for lower body.
Since she is 15, she needs to make VERY VERY sure form is 110% perfect. Don;t go too insanely heavy - she IS still growing. I am assuming she will be on a very balanced diet - nothing restrictive or extreme. She needs all the nutrients she can get right now since she is still building a healthy body.