Less is more. I would encourage her to try throwing with 2 and see how it goes. However, I think it is unreasonable to expect an immediate increase in speed but it will come over time. I have no tangible data but I would guess that the majority of HS age pitchers throw with 3 fingers. Not many girls ever have the hand size to throw with 2 fingers. If they can they are blessed and should use the gift.
I believe that some girls can't and shouldn't move to a 2-finger grip. But, if she has long fingers, try it. Two of my three pitching students use the 2-finger for dropball-fastball. One of the two has VERY long fingers, and it really gives her the advantage
Four fingers, push ball into palm, don't load the wrist. Sometimes a great way to throw an off-speed pitch is by doing all the "wrong" things. I teach a change up that no matter how hard they try they cannot throw it fast.