Friend right now is trying to en passant me with a king. I'm saying that its not legal.
He won't believe me.
He is saying that any opposing piece can capture the en passant pawn.
An en passant capture is a move that involves pawns, exclusively. In particular, the king is not allowed to capture en passant.
From Wikipedia (link):
It is a special pawn capture, that can only occur immediately after a pawn moves two ranks forward from its starting position, and an enemy pawn could have captured it had the pawn moved only one square forward. The opponent captures the just-moved pawn "as it passes" through the first square. The resulting position is the same as if the pawn had moved only one square forward and the enemy pawn had captured it normally.
First of all, neither a rook nor a knight can make the single diagonal "en passant" move. Of course, kings, queens, and bishops can, so the question is why they do not.
A pawn moving two squares represents a group of lightly armored foot soldiers moving at "double time" (speed, actually). In an en passant move, they are "ambushed" by similar light infantry able to give chase at similar speeds. "Kings" and "queens" represent heavily armored "guards" troops (think bodyguards), commanded by the king and queen respectively. These are heavily armed (and armored) "formation" troops that are not good at hot pursuit. Bishops have been likened to artillery or archery, neither of which are good at chasing lightly armed peasants over broken ground. That's why only fellow pawns can use the en passant move.
any piece that guards that square could in principle capture the pawn on that square.
You're incorrect. I've found numerous references that says that you can't capture en passant with nothing but pawns.