Forgive my noobness!
My opponent has the card 'Flood' : Tap target creature without flying.
I entered my attack phase, said what creatures I was attacking with, and tapped the cards. My opponent gave it a few seconds and then used Flood to tap one of my attacking creatures (even though i had tapped it to attack.) My opponent did this to stop the creature from attacking him.
Was this a legal move? After i had tapped the card to attack him, surely its too late for him to tap it? Should he not have tapped it in my initial main phase?