OK, let's say that the following analysis by
MGMAT is correct:
"Possessive nouns are particularly dangerous on the GMAT. Consider the following:
Jose's room is so messy that HIS mother calls HIM a pig.
The possessive noun in this sentence is Jose's. Possessive pronouns can refer back to
possessive nouns. Thus the possessive pronoun his refers back to Jose's. However, subject
and object pronouns may NOT refer back to possessive nouns. Therefore the object
pronoun him is used incorrectly because it may not refer back to Jose's. Subject and object pronouns may only refer back to subject and object nouns. Him would only be accurate if it referred back to the word Jose.
Even though it seems obvious that him refers to Jose, the sentence must be changed in
order for it to be grammatically correct on the GMAT. We can fix the sentence by keeping
his and eliminating him.
Jose's room is so messy that his mother calls Jose a pig."
If that analysis is correct, then "The manager's obstinate and surly attitude prevented his staff from befriending him" has to be wrong for exactly the same reason. The object pronoun "him" is trying to refer the possessive noun "manager's".
MGMAT can't have it both ways.