I think the thought process is pretty much what I outlined above. Your average JW who actually cares about doctrine believes that allowing oneself to entertain ideas critical of the WTBTS and their teachings is sinful and extremely dangerous to one's spiritual well-being. Adam and Eve were perfect people. Despite this, Satan was able to deceive Eve and convince Adam to follow suit and disobey God. If Satan could deceive perfect human being, how easy would it be for him to deceive us mere mortals?
With this in mind, you learn to quickly disregard critical points of view. The WT likens these thoughts to poison that can infiltrate our system and destroy us spiritually. Apostates are villified, regarded as despicable, and hold the lowest rank in the JW hierarchy. The supersticious are led to believe that entertaining apostate ideas opens the door for the Satan's demons.
The most you can do is add to the list of doubts the person refuses to entertain and hope that if/when that particular individual is ready to think for himself, he will look back and take your comments into consideration. The bottom line is, if a JW is a firm beliver and locked into the the WT's mind control, there's precious little you can do. A JW's mission is not to critically consider all the evidence and reach the most logical conclusion. It's to be a zealous advocate for the WTBTS.