I say trust your ear over either dogma. "A friend of Bill's" probably is better, except in the Clinton-era coinage, but it's not a must. The following over-the-cliff application of the principle, which appeared recently on the front page of my newspaper, shows what can happen if you treat it as mandatory:
Lanier has long been a favorite of Ramsey's, who gave her key patrol commands and later put her in charge of the bomb squad, SWAT team and other special units.Extra credit if you spot the other problem in that sentence. Extra extra credit if you also spot the thing that people who are wrong about such things would wrongly call a problem.