"She is up early" is not good enough?
Sie steht frue auf means she gets up early. Your abbreviated sentence does not allow for the verb "steht".
I suppose that'd be "Sie ist früh auf" (i.e. it's [unusually] early in the morning, and she is up and about already; we're not mentioning getting up, it's possible that she hasn't slept at all).
Why is "she rises early" not accepted?