I was playing a game at a coffeehouse not long ago and my opponent got confused on en passant. I think he tried to play it when it wasn't legal. We just called over another player to settle it (don't let them call over someone else who doesn't know the rule!).
If it's a tournament, just call over the tournament director. If you're playing a non-tournament game at a club or something, there's probably a book lying around that you can show him that explains the rule to him.
I'd resign and say something about my opponent being an expert and too smart for me. Then I'd go find somebody else that would explain it to him. He'd feel about 2 centimeters tall.
You are playing a game OTB and you play en passant. Your opponent freaks, "that isn't legal he screams!"
What do you do?