Opponent has ~25 points, I have ~10 points, there are ~25 points left on the board. Opponent went basically Death Cart-Big Money with Forager support and has slowed down, I've just gotten Followers and Princess from the Tournament pile and hit him with Followers, having taken all the Saunas and four of the Avantos.
Opponent: "Please just resign"
Me: "Unless you have a definite Province on the next two turns, I have plenty of room to win."
Two minutes later, with Duchies and Estates gone, opponent has the opportunity to win by buying the last Avanto, but doesn't notice it at first, and asks for the undo after buying something else.
Because of the "just resign" comment, I denied it. What would you have done?