Whom did David send back to Jerusalem to frustrate the advice of Ahithophel?

Answer: Hushai the Archite

Hushai pretended loyalty to Absalom, and Absalom chose his advice over Ahithophel's, for the LORD had purposed to thwart Ahithophel.

“When David came to the summit, where he used to worship God, Hushai the Archite was there to meet him with his robe torn and dust on his head.”— 2 Samuel 15:32 (BSB)