Book of Eclipse
Change all face-up monsters on the field to face-down Defense Position. During the End Phase, flip all face-down Defense Position monsters your opponent controls face-up, and your opponent draws 1 card for each.
If the opponent's monster is flipped effect, it is activated. Effect monster's effect can't be activated unless it's faceup. Since the card effect never state flipped up monsters effect can't be activated unliked Ceasefire which specify flip monster effect not activated at that time. Yes, opponent can use their monster effect if their monster are flipped face-up.