I am absolutely with Holt CJ here! He is clearly experienced in such legal issues.
What if the line colleagues, when asked, just simply don't happen to like the individual or are jealous of him? Where is the evidence other than a gut feeling? In aviation ideas of someones style are often generated by myth and rumour. Individuals can get a bad reputation simply by making a silly operational error one day when they are normally totally competent and their co-pilot just happens to be a big mouth.
Our chief pilot friend above is indicating that such a reference might prevent that person getting work with his airline. Surely that his totally illegal - and for very good reason.