What a leaky duct?
I had a guy last week trying to convince me to unleash unlimited amounts of water and chemicals through an office building, 40 foot horizontal to a 8 story drop to a 30 ft' horizontal, none of it accessible with out opening up walls. I told him I would only attempt it if he signed a waiver saying I was not responsible for water damage.
He said "Don't worry, I used to install that kind of ductwork and it is water tight” He looked at me like I was crazy when I said I have never seen one that didn't leak somewhere.
I figure if he is so sure it won't leak he should have no problem with a waiver. I don't think flooding it will be very effective anyway. I think he is just looking for something that says it has been cleaned to get his boss off his back.