In the past, as I recall, we’ve always just done it where you take the bomber out of the battle. Then if the battle hasn’t started yet, the attacker decides if he wants to call off the attack or continue without the bomber. If there have already been rounds of combat that have been rolled, the defender gets to choose whether or not to re-rolls the round(s) of combat that have already occurred. If the defender decides to re-roll, then the attacker would get the chance to call off the attack. We would never let the bomber go help another battle that turn, or redo any other battles that had already occurred.
This is generally the most fair way of doing things, but there are always special circumstances that can happen and in that case you just negotiate with the other player until you come to a fair agreement. If you can’t come to an agreement about a fair way to resolve the mistake then you probably shouldn’t continue the game anyway.