WooCommerce - Send an email from the customer on order complete.

gistfile1.txt

add_action( 'woocommerce_order_status_completed', 'custom_send_email_from_customer' );

function custom_send_email_from_customer( $order_id ) {
    $order = new WC_Order( $order_id );

    $headers = 'From: ' . $order->billing_first_name . ' ' . $order->billing_last_name . ' <' . $order->billing_email . '>' . "\r\n";
    wp_mail( 'someone@somesite.com', 'Subject', 'Message', $headers );
}