It's definitely out there somewhere... I might have read it in an FAA legal interpretation, though I don't think so, as it is not illegal. Where I saw it was more likely in an accident report docket, so that would be like finding a needle in a haystack. At any rate, as I recall the PIC was pointing the finger at his pilot buddy working the radios/trying to deflect and throw his buddy under the bus... The FAA came back and said no dice; it only takes one person to fly that airplane, and that the PIC is 100% responsible. Which we, of course, know.