Some parts have already been mentioned by others, but this is what I'd do...
Two minutes would be enough to read these verses from Deuteronomy chapter 18:
20 “‘If any prophet presumptuously speaks a word in my name that I did not command him to speak or speaks in the name of other gods, that prophet must die. 21 However, you may say in your heart: “How will we know that Jehovah has not spoken the word?” 22 When the prophet speaks in the name of Jehovah and the word is not fulfilled or does not come true, then Jehovah did not speak that word. The prophet spoke it presumptuously. You should not fear him.’
and then point out that "God's" only true and spirit directed organisation on earth was wrong about the end of this system and/or the preaching work in 1914, 1915, 1925, 1941, 1975, within' the 20th century
and then ask: The WT was wrong at least six times! According to these deuteronomy verses... how many times do you think a prophet could falsely speak in the name of Jehovah before it would become clear that he is a false prophet? New light would never conflict with such important dates coming from jehovah's own revealed timetable.
If there's some time left, I'd add this bonus:
John 8:44 clearly states that the devil is a liar and the father of the lie.
With a long history of so many failed prophecies, how would the WT be an exception in the light of this inspired word? Maybe read while the other was still thinking...
1 John 4:1 Beloved ones, do not believe every inspired statement, but test the inspired statements to see whether they originate with God, for many false prophets have gone out into the world.