Any time we have a banned card pop up, we just have them exile it and draw a card. If he tutored for it but it wasn't a tutor that reveals the card, maybe you just restart the game with legal decks - you said he planned to use it on turn four to tutor Dark Depths/Thespian's Stage, so I assume you were on turn two or three. No big deal to restart.
2023 Average Peasant Cube|and Discussion
Because I have more decks than fit in a signature
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