Why not walk away following one last blow....
"dear mr elder, would you afford me the time of answering just one question? You tell me you have the truth and that I am seeking it. Can I ask you just one theological question?"
if he says yes..
"thank you...... What is gods name?"
if he is smart he will admit it is not Jehovah, ask him therefore if it is true or false to call god Jehovah and preach that name to people? . Ask if the average jw knows that this is not gods name but a misunderstanding of a text etc.... Explain the danger of his last reply, assuming that once you find truth, it is no longer important to seek it, if one went by Wt literature alone you would be deceived into thinking Jehovah was gods name.
assuming he is not smart, you can swing your final blow before walking away. I assume you know the history of the mistaken transcription Jehovah? The masoretes some 1000yrs after Jesus added the vowels from Adonai (lord) under the sentence YHWH (he causes to become) because they felt YHWH was too sacred to say. Adonai was written beneath it so that people said lord instead of YHWH. Centuries later idiots had wrongly mixed the vowels into the sentence, Latin changed YHWH into JHVH and the vowels from Adonai were added JaHoVaH.
Once scholars realised the mistake when they saw the original masorete texts with Adonai written below the sentence YHWH, it was removed from bibles and hyms etc as an embarassing mistake...... Until the genius Rutherford arrived on the scene and put it back lol.