Forum:Karakuri Trick House Help

My friend runs Karakuris but we always have an argument about Trick house.

Example: I attack his Karakuri Soldier mdl 236 “Nisamu” with my Scrap Dragon. He activates it's effect and switches it to defense mode. Then He activates Trick House and destroys my Scrap Dragon.

The Question is can he activate it? My guess is Since he has let the karakuri be selected, he has to let the attack resolve after the effect resolves. His guess Right Timing means I can Play it. Please help me find out the truth.