How old is she, how big is she, what level does she play? If you are asking about bats and drop-weights, I'm assuming that she isn't at the travel level yet. (?)
Very few travel players (at least in California) are unable to swing at least a -10 effectively, but that's not true with respect to rec leagues, where many players swing bats in the -11 to -13 range (i.e., lighter relative to their lengths).
You don't want her swinging a bat that is too heavy for her or she's going to suck, and, in the immortal words of Karl Spackler, "
[s]he'll quit the game."
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