Forum:Gravi-Crush Dragon

I had a question about this card, I was playing my friend when he used this, and he says he can activate the effect whenever he wants, as many times as he wants and I wanted to know if this was true. I played a monster on my turn, and he activated the effect destroying my monster. I used a magic card afterwards to special summon a monster, and he used the effect again to destroy that monster. Is this how the effect works?