Is your setter consistently setting to tight or way off the net?
There may be more to it than “squaring-up.” Not that squaring-up doesn’t have a very important place in setting (and in volleyball, generally speaking).
But, for this post’s sake…
Take a look at her feet. If all of her sets are tight, tell her to try to step forward more with her right foot. And the opposite is true. If all of her sets are too far off the net, she may be stepping too far forward with her right foot.