I've understood the interpretation of en passant capture to mean the
capturing player makes the en passant move 'as soon as legally
available'. For example, if the initial double-step pawn move results in
a discovered check, the check must be dealt with, then, on the next turn,
if the player is not again in check, the pawn may be taken en passant, if
that move is still available. A series of checks would 'push' the en
passant capture along with it. The checks could even be ended by the
double capture move originally suggested. If that move, the en passant
capture ameliorating check, was available, then it would have to be taken
then, or the en passant opportunity would be lost. This could
theoretically occur in a FIDE game, no? Anyway, the en passant capture
would then be available to the other player during his next move, which
would have to be the one-move capture, and not the two-move non-capture.
In which case, the situation described would be a serious blunder, or a
brilliant sacrifice.
This does not hold if Roberto is strictly right, and there is a voluntary
pass by the opponent, for, theoretically, the opponent could have,
instead, made a voluntary en passant capture between the non-capturing
moves.