Can you capture a pawn en passant with a king? 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.