After David condemned the rich man in the parable, what did Nathan tell him?

Answer: You are that man!

Nathan declared, "You are that man!" (KJV: "Thou art the man") and exposed David's sin against Uriah.

“Then Nathan said to David, “You are that man! This is what the LORD, the God of Israel, says: ‘I anointed you king over Israel, and I delivered you from the hand of Saul.”— 2 Samuel 12:7 (BSB)