Proper Courser of Action

You are the head and only judge in a Theros/Khans of Tarkir Standard Grand Prix Trial (GPT). You are watching a game between Alex and Nathan. During Nathan’s turn, he casts Hero’s Downfall targeting Alex’s Courser of Kruphix. Alex puts the Courser in the graveyard, but does not turn the top card of his library face down. The players proceed through combat before Nathan passes turn. Alex untaps, draws the top card of his library and then proceeds to flip the next card in his library face up. He immediately realizes what he’s done and calls for you. What do you do?

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While it is not technically correct for Alex to leave the card revealed after Courser of Kruphix has been destroyed, it is generally accepted to be ok to do so. However, once he draws that card, by revealing the next card for a now non-existent Courser, Alex has committed Looking at Extra Cards and will receive a Warning. The random portion of the deck needs to be randomized. Determine if Alex has put any cards on the bottom of his library, such as with Scry, since the last time it was shuffled. If there are known cards on the bottom, set that many cards aside, have Alex randomize the rest of the deck, present to Nathan for a shuffle, and then replace the set aside cards on the bottom of the Library. Then, if necessary, issue an appropriate time extension for the players and record the penalty on the result slip. It is also worth noting that because the Game Play Error was caught immediately, it is not correct to give Failure to Maintain Game State to Nathan.