...

foreach( $referrals as $referral ) {
    $order_id = $referral->post_id;
    $order = new WC_Order( $order_id );

    // added customer info ...
    $output .= '<div class="customer-details">';
    $output .= 'Customer: ' . $order->billing_first_name . ' ' . $order->billing_last_name;
    $output .= '</div>';
    // ... added customer info

    $output .= '<table class="shop_table order_details">';
    $output .= '<thead>';

    ...