I would show them a copy of a patently false statement published by the WT such as the following:
"True, it is expecting great things to claim, as
we do, that within the coming twenty-six years all present governments will be
overthrown and dissolved; but we are living in a special and peculiar time, the
`Day of Jehovah,” in which matters culminate quickly; and it is written, `A
short work will the Lord make upon the earth.’… In view of the strong Bible
evidence concerning the Times of the Gentiles, we consider it an established truth that the final end of the kingdoms
of this world, and the full establishment of the Kingdom of God, will be
accomplished at the end of A.D. 1914." The Time is at Hand (1903 edition), pp. 98-99
After they have read it, I would ask:
1. Did the men who published that think it came from Jehovah?
2. Did it really come from Jehovah?
If they say that is too old, you can:
a. Point out that the Bible is thousands of years old.
b. Follow the same procedure with hundreds of other asinine WT statements quoted on jwfacts.