Play with her mind some...see if she can hit the back fence off one bounce...doesn't matter if it's in or out. Then, once she is able to do that ask her why she can do it. She should figure out that it is a combo of striking more behind the ball with a slightly different grip. If she doesn't, repeat the drill a few times til she does.
It will take some time to break the habit. Be patient with her and encourage her to have patience with herself.
Might want to also set up a video camera behind her. Show her what she is doing.