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Admittedly, the son made a common rookie mistake in listing the LW as a reference without checking first. But I would think that the letter writer should recuse himself anyway, regardless of the son's performance or idiosyncracies, simply the LW's boss is the applicant's father. If I were a potential employer, I certainly would not trust a reference from someone who works for the applicant's father. Yes, call the son and explain the requirements of business etiquette, but the reason not to give any more references is as simple as, "I can't give you a reference because I work for your dad." And simply mentioning this fact to anyone else who does call to check a reference might be enough to end the phone call.