In George Orwell's Politics and the English Language, he advises writers to "avoid passive voice whenever possible."
Who is this "he"? Probaby Orwell, but there is nothing in the grammar of the sentence that guarantees that. In fact, the example mixes two possible sentences,
Structure #1: George Orwell's
Politics
and the English Language advises writers to "avoid passive voice whenever
possible." [The unclear "he" has been eliminated and the essay itself is
now the subject of the sentence.]
Structure #2: In Politics
and the English Language, George Orwell advises writers
to "avoid passive voice whenever
possible." [The unclear "he" has been replaced by "George Orwell.]
The way to fix mixed constructions is to decide what constructions have been mixed in the first place and choose between them.