If she has a mechanically sound swing, does she really need a -13? If a -13 gives her really good bat speed, move to a -11. You may drop down a ranking to only "good" bat speed, but the extra mass of the bat should make up for it. You may even see less mishits as the bat won't deflect off the ball as much.
Most likely she doesn't need a -13, but I cant say that for sure.
I grew up in the age where you most likely used team bats, and whatever fit best you used haha