In the sentence It will be just she and the stars what pronoun does the adverb just require she or her?

Although grammar rules suggest that "she" follows a linking verb, her will almost always be used.

If "just" were a preposition, then the proper form would be her, as its object. But just is just an adverb. You could say "it will be her and the stars" with the same meaning, and with the same decision: writing correct grammar or writing the way modern English is spoken.