Which seer asked King Jehoshaphat, "Should you help the wicked and love those who hate the LORD?"

Answer: Jehu son of Hanani

After Jehoshaphat had joined Ahab at Ramoth-gilead, Jehu son of Hanani rebuked him, saying the LORD's wrath was upon him.

“When Jehoshaphat king of Judah had returned safely to his home in Jerusalem, Jehu son of Hanani the seer went out to confront him and said to King Jehoshaphat, “Should you help the wicked and love those who hate the LORD? Because of this, the wrath of the LORD is upon you.”— 2 Chronicles 19:1-2 (BSB)