As serious as the original poster :doh:
His plate was missing by admission. I don't give a fudge what his neighbors housemate did or did not do.
The point is that this kind of tit for tat shit is kindergarden and gets old quickly. I'm not saying you have to actually like your neighbors, but you better come to an understanding because they can make your life hell if they please. And they can do it legally.
So unless he enjoys this kind of constant irritation, he will bolt his plate where it meant to be, wash his fawking truck (if that's what it takes), walk over, talk with his neighbor, and come to some sort of an agreement.
Life is way too short for this kind of shit.