Eugene Volokh is right to question the claim that the Morrison sentence is ungrammatical. The sentence as written is grammatical. Compare:
1a Toni Morrison's genius enables her to create novels [].
1b Sam's ax enables him to cut down trees.
1c Bob's hair enables him to sleep.
1d NP1 enables NP2 to VP.
All of the above examples are equally grammatical because they are all based on the prototype sentence (1d). If you accept (1b), as I believe most English speakers would, then the Morrison sentence (1a) is grammatical -- just as the nonsensical sentence (1c) is, or any sentence on the pattern of (1d), where NP is any noun phrase and VP is any verb phrase.
There is no rule in English grammar -- nor in any grammar that I know of -- that makes it illegal for the antecedent of a pronoun to be in a different case than the pronoun. Such a rule would make it nearly impossible to use pronouns in an inflected language, because you would be obliged to re-introduce the antecedent in the appropriate case every time you needed to use a different case.
If you're not convinced of that, consider the following sentences:
2a Susan's car hit Bob's so Bob punched Susan.No English-speaker rejects 2b, which obviously communicates the same meaning as 2a but uses prepositions for the second reference to Susan and Bob.
2b Susan's car hit Bob's so he punched her.
Ultimately, this is not a question about grammar but rather about syntax, or tyle and usage, which I do not find terribly interesting. As somebody who actually studied grammar, however, I can't resist weighing on that part of the controversy.