wpae_full_customer_name.php

<?php

/**
 * Get Full Customer Name from Order ID
 * 
 * Author: Zeshan Ahmed
 * Author URI: https://zeshanahmed.com/
 */
function za_full_customer_name( $order_id ) {

  $order = new WC_Order( $order_id );

  $user_id = $order->get_user_id();

  // Check if the user id is not false. 
  if ( $user_id ) {
    $fname = get_user_meta( $user_id, 'billing_first_name', true );
    $lname = get_user_meta( $user_id, 'billing_last_name', true );

    // In case customer info does not have 'first name'
    // get the name from the order billing info.
    if ( ! $fname ) {
      $fname = $order->get_billing_first_name();
    }

    // In case customer info does not have 'last name'
    // get the name from the order billing info.
    if ( ! $lname ) {
      $lname = $order->get_billing_last_name();
    }

  } else {
    // Get the billing first name
    $fname = $order->get_billing_first_name();
    // Get the billing last name
    $lname = $order->get_billing_last_name();
  }

  $name = $fname . " " . $lname;

  return $name;
}

?>