I would ask them "Who runs the organization"?
If they say Jehovah, then I would point them to the various scriptures where it says "to test the inspired expression to see whether it originates with GOD", "Make sure of all things, hold fast to what is fine", "If a person comes to you in my name and utters a prophecy that does not come true, then I did not send them", or remind them of when Paul visited the Bereans and witnessed to them, how they didn't simply take his word at face value, but searched the scriptures carefully to see if what he said was true.....and he commended them for it.
Time and again in the scriptures GOD tells us to search for the truth. To keep knocking and it will be open, to keep searching. This is a lifelong process and doesn't end with the WTS
If the person says the GB runs the organization, then point them towards the Book of Acts where the society tries to use the one instance as the sole authority for their existence. Then point them towards 1st and 2nd Corinthins where that congregation wrote to Paul for advice concerning the man who took his father's wife into his bed. Why didn't they write to the GB (if indeed there was one)? Why didn't Paul in all his missionary journeys ever report to a GB? Why is there no reference to a central body of elders with authority over the 1st century Christians? What about in Revelation where Jesus counsels the seven congregations but makes no mention of a GB? Which had there been one in existence, they would have utterly failed in their duties for the fact that Jesus gave such strong counsel.
IF they insist that there was a GB despite pointing this out, then conclude that if things could get messed up back then, then why not now? Consider also that the Apostles argued amongst themselves while Jesus was alive and still had problems after his death as shown in Acts. Why is this incomprehensible now?
Peace,
Awen