'Jehovah' just ain't there.
I actually have my Interlinear Translation handy for just this occasion. (I only have newer literature, like 1980+, so this was the only thing I'm intentionally keeping).
YOU MUST GET THE INTERLINEAR TRANSLATION IF YOU WANT TO ATTACK THEM.
Else, they'll do as others said, dodge the issue, 'but but but' you, etc. If you can show them DIRECT proof of translation errors, that might open the door.
Here are some freebies:
Compare Rom 10:9 vs Rom 12:19 (same book!) How is Kupios translated in one place, and then the other? Same word, different meanings?
Compare 2 Thes. 2:1 vs Luke 2:9. How is Kupiou translated in one place, and then the other? Same word, different meanings? What would happen if they were both translated the same?
Jesus uses the phrase 'Truly I say to you...' over 50 times in the New Testament. In EVERY case, the NWT translated 'Truly I say to you, (COMMA!), blahblahblah'. See John 3:2, John 5:19, Mk 8:12, Mk 9:1, Mk 9:41, Mk 10:15, Mk 14:9, Matt 6:2, Matt 10:15, Heb 4:7, Luke 12:37, Luke 18:17, Luke 18:29, Luke 21:32, etc.
It's important to use the Interlinear here, so he can see that the original Greek is EXACTLY THE SAME in all cases. Now, compare the original text of Luke 23:43. And compare how the WTBTS translated that ONE instance of 'Truly I say to you'. How does the placement of the comma change the meaning of the scripture? Why do you suppose they made that change?
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In any case, the above attacks two of the WTBTS primary teachings, and shows how they doctored the bible to match their beliefs. If this doesn't open the door - proving the WTBTS false using their own literature that your father undoubtedly has in his own house and is still considered 'accurate' - I don't know what will.