Rangers Get a Bonus on Spot and Listen Checks against Favored Enemies

The characters are moving through the dense jungle listening to birds, reptiles, mammals, and sounds they cannot identify.  The Dungeon Master (DM) asks each character to roll spot and listen checks, for the party is secretly being followed by Drow enemies.  The players and their characters, are unaware of the pursuing Drow. Only the DM knows the Drow advance.  But the DM knows there is a chance
the party will hear some of the Drow if a party member listens well enough and if the DM's roll for the Drow to Move Silently does not beat each character's listen roll.

The characters roll.

The DM rolls. 

The DM compares the numbers and realizes that none of the character's heard the Drow.  Not yet.  But one of the characters only missed the check by one point.

[break] - Unbeknownst to both the players, and the DM, is that rangers get a +2 on spot and listen checks against their favored enemies.  The DM is unaware of this rule, and may not even recall that Drow are a favored enemy of the ranger in this party.  If the DM had declared that the party was being stalked by Drow to the players, the player of the ranger may have pointed out that his ranger
should get an additional +2 listen bonus; but we can't expect the DM to reveal everything that is coming upon the characters because this would take some of the fun out of the game.  [break]

The characters continue, unaware of how close the Drow are.  The Drow attack the party gaining the element of surprise.  Several party members are wounded greatly by the Drow's surprise attack. The Drow win initiative killing a party member and wounding more.  The party responds by fighting back, and eventually, after two rounds of intense melee requiring two hours of gaming time, the Drow are driven away.

The players, on behalf of their party, begins to wonder if there is any way to save their dead comrade. Could they have done something, or could they do something now to reverse his death. 

Only now does the player of the ranger recall the +2 bonus he should have received on his listen check
to the Drow, and how well he rolled, and that it might have been enough.

What should the DM do?  What would you, as DM, do?  Would you roll back the battle and undo hours of play time and replay it all?  Would you declare that the statue of limitations for corrections was one round and that it is too late?