Final answer:
The sentence has a minor error with possessive apostrophes. 'Westerhazy's' should be 'Westerhazys'' and 'Graham's' should be 'Grahams'' when referring to the land and the pool owned by the respective families.
Step-by-step explanation:
The sentence "He went through a hedge that separated the Westerhazy's land from Graham's, walked under some flowering apple trees, passed the shed that housed their pump and filter, and came out at the Graham's pool" is almost correct but has a minor possessive apostrophe error.
"Westerhazy's" should not have an apostrophe since it refers to land owned by the Westerhazy family, not one Westerhazy individual, making it plural possessive. Additionally, "Graham's" before "pool" is incorrect if referring to the pool belonging to the Graham family; it should not have an apostrophe.
Corrected, the sentence should be: "He went through a hedge that separated the Westerhazys' land from the Grahams', walked under some flowering apple trees, passed the shed that housed their pump and filter, and came out at the Grahams' pool."