I didn't really know that someone already made this up but there are two meanings to this.
A. Someone who does the deeds of Jenova of the church of the one winged angel, a group of geeks who are obsessed with a video game.
B. This one I made up myself. Someone who's Played FF7 to the point that you fight either the first Jenova, or the Final.
Jim: You see that dude over there? They say he's a Jenova's Witness.
Jesse: Huh? What the **** does that mean?
Jim: I don't know, but I find it funny.