When is an action acknowledged?
My opinion is that it's to late for NAP to go back and think about a possible response. He agreed on resolving his opponent's spell by moving the card from the battlefield toward the graveyard. He had all the time he needed to think about a response before starting to move the enchantment.
Moreover, I think that what AP said (“it is now a 1/2 with 2 damage, so it should die”) it's kind of information NAP gained by his opponent due to his reaction. I don't think it would be fair to let him go back on his decision. :)
My two cents.