Modify existing code to show user nickname in woocoomerce order email instead of user_login

To show the users nickname in WooCommerce order emails instead of the user login, you need to retrieve the users display name from their user data.

add_filter( 'woocommerce_email_customer_details_fields', 'add_user_nickname_to_woocommerce_emails', 10, 3 );

function add_user_nickname_to_woocommerce_emails( $fields, $sent_to_admin, $order ) {
    $user_id = $order->get_customer_id();
    $user_info = get_userdata( $user_id );

    // Retrieve the users nickname (display name)
    $user_nickname = $user_info->display_name;

    // Add the nickname to the email fields
    $fields['user_nickname'] = array(
        'label' => __( 'Nickname', 'woocommerce' ),
        'value' => $user_nickname
    );

    return $fields;
}