I like to have players think of their rally ball as being one which allows them to maintain consistency whilst neutralising the opposition (at least stopping them attacking). Therefore the rally ball will need to vary a little depending on the opponent, but typically around 80% of maximum is around the right mark to aim for.
She can work on rally tempo drills to improve this. I tend to use a gearbox analogy to help people understand how they can vary their tempo. Start off in first gear, which is basically as slow as she can hit the ball and still make it over the net. Then go straight to 5th gear, which is 100% as hard as she can hit - this establishes the two outer limits and frames the drill. Then go back to first gear and every 5 minutes or so, change up a gear. Around 3rd or 4th will be the right tempo for your (I mean her) rally ball.
Hope that makes sense!