This "prophecy" relies on setting the year 1914 as Christ's second invisible coming. 1914 is based on a historically inaccurate date of Jerusalem's destruction in 607 BC by the babylonians. If (and that's a big IF) you can prove him that 607 is incorrect, then 1914 is incorrect, 1919 is incorrect, and the whole religion is nothing more than a group of power hungry men leading blind people.
Ask him to prove from outside the Watchtower that 607 was indeed the time when Jerusalem was destroyed. There are absolutely no historians (except dodgy JW ones) that would have a doubt that Jerusalem was destroyed in 587/586 BC. Your friend's main line of reasoning will be that the history is inaccurate, the Bible prophesised that Jerusalem will be destroyed for 70 years, and from historical records (yep, I'm sure you already picked up the weirdness of all) we know that the end of the jewish exile was in 539 BC when Darius freed them, and after 2 years in 537 BC they got home, ending the 70 years period the Bible foretold, so it must have started in 607.
The most obvious error in this is that the exact same sources that say 539 BC also say 587 BC as Jerusalem's destruction, so you can not just say one of the date is not reliable, and the other is. JW's have no problem with that anyway...
This is a major issue for us, ex-JW, so you can find a huge amount of resources on the net, including explanation how the 70 years prophesised doesn't conflict with the historical records.