Ames is playing against Netty at an Ixalan Sealed PPTQ. Netty is tapped out when Ames untaps on his turn 5. Ames draws for turn, then immediately uses one hand to tap his 4 lands, and the other to put a land and a Charging Monstrosaur into play, both tapped and saying “Attack for 5”. Netty says “I guess I take the 5.” As life totals are being adjusted, Netty notices that the land Ames played was a Stone Quarry, and calls for a judge. When you ask Ames, he says that he was playing quickly to conserve time, and had mixed up the cards in his hand, playing the wrong land. When you check, the rest of his hand is three Mountains.
What do you do?