Steve Hedley, the Assistant General Secretary to the RMT, refused to confirm to Ferrari whether a tube driver had failed a drink driving test and instead launched into what Ferarri called a "vile and offensive" rant.
"You need some form of media training" said the stunned radio presenter, after Hedley interrupted questions and asked repeatedly: "Have you stopped beating your wife?"
The pair were discussing why the union is striking over a driver who was reportedly sacked after failing an alcohol breath test. Hedley said the driver was "not drunk" but has diabetes, which affected the test result.
Ferrari asked Hedley to confirm if the driver had indeed failed the test, but an indignant Mr Hedley said Ferrari should "apologise for that statement" and began to shout: "Have you stopped beating your wife?" to Ferarri's bemusement.
"Have you stopped beating your wife?" is a colloquial phrase, used to suggest that the person being asked a question thinks it is unfair and presupposes guilt.
Whether the respondent replies "yes" or "no", they are effectively admitting to beating their wife.
But Ferrari was seemingly not aware of the phrase, and appeared to believe Hedley was accusing him of domestic violence. Ferrari answered curtly: "I'm a divorced man" and continued to try to press Hedley on whither the tube worker did indeed fail the drink driving test.
"So the London Underground have fabricated the evidence that he's failed a driving test?" Ferrari asked, as Hedley continued to avoid the point and interrupt him.
Ferarri eventually cut Hedley off, saying: "Goodbye Mr Hedley. I just find that offensive.
"I have tried time and time again. I have to take it that your colleague failed. You had really interesting points to make there. Clearly you are unhappy with the test. Clearly, it may have been poorly administered.
"Your colleague, I think, appears to have diabetes. If he is that and that's why he failed his test, I will be there to support him.
"But your vile and abusive stance means I never wish to speak to you again."
"And in my eyes, you have diminished the case of your colleague immeasurably. For you, as an Assistant General Secretary, you need some form of media training. Goodbye."
In a bizarre twist, in 2013 Hedley was himself accused of domestic violence by an RMT colleague whom he had a relationship with. The RMT carried out an internal investigation and found he had “no case to answer”.
Hedley wrote in a blog post that the women abused him during their relationship: "During the 18 months I spent with Ms Leneghan I was slapped, deprived of sleep, punched, pinched, scratched, elbowed in the ribs, kneed, had shoes ,belts and other objects thrown at me, had paint rubbed in my face and was kicked in the stomach about a week after I had an operation for a hernia."