According to the Cambridge Grammar of the English Language, there are no non-finite constructions in which a subject is required (p1175).
The part in question for her to write is a to-infinitival clause which is non-finite and so does not require an overt subject. The only reason to include the subject would be to avoid confusion, or for emphasis.
Seeing as, in the context given, the only possible subject that could be understood for this to-infinitival is Toni Morrison, it seems you're safe leaving for her out.
An example of a similar construction with subject omitted:
Much of the improvement which has been, of late, discernible in this
class of productions, is to be fairly ascribed to the spirit which he
has awakened, and to the models which he has afforded. He has made it
easier to write a good oration than it formerly was, and has also made
the public less tolerant of indifferent ones. (North American Review: January 1837: 138-153)