En Passant Checkmate I made a check with a pawn that was subject to an en passant movement. 

The check led to a checkmate, but the rules at chess.com didn't count it as a checkmate but let the opponent make his move. He took the pawn with his pawn with an en passant movement. 

It seemed a bit strange to me because, I thought that in this case the check mate would be valid, and I wonder what happens in the case the opponent plays some other piece, does the checkmate become valid? 

Or he is obliged (no other move allowed) to make the "en passant" movement even in the possibility he is not aware of the en-passant type of movement? (In this case I suppose the game is won, anyway)

My idea is that the rule is wrong in this case, the check mate should be counted as valid even if the idea of the en passant is that the movement ends at the time the opponent plays, but I think in this case the king is dead.
It is not checkmate if the other player has any legal move that gets them out of check. Capturing the checking piece is one such way; whether the capture is en passant or not is irrelevant for the purpose of this question.

In this case, an en passant capture is the only legal move. If the other player didn't know the en passant rule, I suppose they might lose on time unless they figure out what's going on first. Using a website such as chess.com one might be able to discover the rule accidentally, since clicking on a piece normally highlights all the possible moves for that piece.

Regarding your question of "what happens in the case the opponent plays some other piece", that would be an illegal move. A site such as chess.com simply won't allow it. In an over-the-board tournament, the consequence may vary: in blitz, the player who makes the illegal move loses; in long games, the move gets retracted and a time penalty is applied, at least for a first offense.