Changing rotation of Quaternion

I have a character moving towards another character, they are both navmeshagents. However when they arrive they don't always look towards eachother. I want one character (the main character) to always turn and look towards the one he is talking to (or is being talked to). What I need to do is retrieve the rotation of one character (npcObj) and the rotation of the player (playerObj), and then do -180.0f on the npcObj's rotation and place this as the playerObj's rotation.

I am very close I believe, but I dont know how to solve it for my problem:

 public static void turnToNPC(GameObject npcObj, GameObject playerObj) {
     Quaternion npcY = npcObj.transform.rotation.y;
     float smoothSpeed = 0.5f;
     Quaternion dir = new Quaternion();
     dir.y = (npcY - 180.0f);
     playerObj.transform.rotation = Quaternion.Lerp(playerObj.transform.rotation, dir, Time.deltaTime * smoothSpeed);
 }