Forum:Mirror Wall ruling

I need a clear answer for this. According to Mirror Wall's lore, it says "The attack of your opponent's attacking monsters is halved." A friend of mine who uses it says a Pojo ruling states that the monsters' ATK will ALWAYS be halved as long as Mirror Wall is face-up, but I say it's only during the opponent's Battle Phase. Who is correct? Is the Pojo ruling a mistake, or did my friend misunderstand it? (a link would be helpful, so I can see if it was clear or vague.)