42.java

// You may do this in PlayerMoveEvent or somewhere else.

// So you have those
final Player player;
final Location targetLocation;

// You get this one
final Location playerLocation = player.getLocation();

// You want the player to look at target

// First get the Vector that goes from the Player to the Target
final Vector direction = targetLocation.substract(playerLocation).toVector();

// Transform this direction Vector to a Pitch/Yaw couple
double pitch = ((player_location.getPitch() + 90) * Math.PI) / 180; // Magic
double yaw  = ((player_location.getYaw() + 90)  * Math.PI) / 180;   // Magic too

// Create a location when the player will get teleported
// It will not move, just change the direction he's looking at
final Location teleportLocation = playerLocation.clone();
teleportLocation.setPitch(pitch);
teleportLocation.setYaw(yaw);

// And teleport the damn Player
player.teleport(teleportLocation);