OCTGN

with the 0.7.4 update, you can look through piles you don’t have control, regardless of if the pile is visible or not. Your opponent can look through your library whenever they want and all the cards are viewable/face-up.

Perhaps the pile’s controller would have a pop-up message that would say “your opponent is attempting to search through a pile that is not visible to them. Allow?”

Or have the client make sure that the pile is visible to that player before they look through it.