This came up in my very first game, and I couldn't find a good answer in the rules (which isn't to say that it's not there).
Scenario:
Active model uses an Attack Action with a Target Number (say, CA 5, TN 10, Def WP).
Attacker flips a "3"; defender (WP 5) flips a "2".
The rules tell us that, in an opposed duel, the model with the lower value has to choose to Cheat first. So it looks like the Defender has to choose whether or not to cheat, on an attack that is currently failing?