Could also be she's coming out of her stance(standing taller during the swing) when swinging, causing her to hit the very top of the ball and topping it. Also I agree with Eric, it could be downward swing, or the exact opposite, a severe upward swing causing her to top the ball right in front of the plate
Have her hit line drives off the tee where she can see trajectory.(long tee) Then proceed to front toss. More line drives. She will figure it out naturally.