Pratt,
Your decision to not bring religion into this issue is a very good one. The instant you even say "religion" or "Jehover's Witness" you will make it a religious issue and bring in all sorts of of "freedom of worship" issues from Human Resources.
Keep referring to the issue as a "personal issue between employees".
If religion does come up, such as when you have to confront the antagonize, make sure you emphasize that you don't care what religion the person is and that all you care about is performance at work (what he does on his own time is none of your concern).
I would let the antagonize know that any issues he might have with the fading JW should stay out of the work place because it is affecting his and the other employees ability to work.