The Manhattan guide states that None should be evaluated along with the "of" construction to determine if its singular or plural.
None of the boys ARE playing. (boys -plural)
None of the money WAS here. (money - singular)

Kaplan 800 question:
As of this morning, none of my friends have been able to solve the puzzle.
Correct answer as per Kaplan is:
As of this morning, none of my friends has been able to solve the puzzle.

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Looks like this contradicts the manhattan guide. Because in the "of" construction "my friends" is plural, which takes the "have been" and not "has been".
Can someone please clarify this apparent contradiction?