Forum:Can you special summon Endless Decay in the oponent turn?

Playing with a friend he do the special summoned Endless Decay right after one of my attacks when he have less than 2000 life points. I say that he can't do that in my turn but he say that he can do because in the card effect description says that a player can special summon this card if have less of 2000 life points and in that moment he had less of 2000 life points. I think that in your turn you can special summon the card but in the opponent's turn you can't.

What is the right interpretation?