The FDS theory/doctrine/prophecy is easily blown out of the water...I've said this to still-in dubs and they hate with a passion what I say to them because it is undeniable.
Ask them do the FDS have the truth and proclaim it? Yes they say...
Ask them so the statement "Millions Now Living Will Never Die" or "The Generation that saw 1914 will see the end of the system of thing" are truth? They were they will say but now new light has been revealed...
Ask them so the old light on the two points that I mentioned, who were the author of those two points? Jehoover or the GB? I.E. Did those old light points have the backing of the Holy Spirit? They will most likely say Jehoover but even if they say the GB you have them....
If the old light was backed by HS and Jehoover then He is a God of Falsehood so bye bye...if the GB are the author of the old light and Jehoover refines the teaching then the JW's are followers of men and the bible warns against that.
Conclude by reinforcing the point by saying that there could be doctrines now they believe that are considered "truth" that will get new light...thus God is a liar or the GB are not backed by HS.
Some might say "Ah yes but the GB are just imperfect men" and that statement is true, as individuals they are imperfect and may say a naughty word when they hit their toe or do a double take when a large breasted woman walks in front of them but as a group, when they come together as the GB/FDS, they can never ever be wrong IF they are backed by HS.
End of.