<?php
function gens_raf_customer_email( $order, $sent_to_admin, $plain_text ) {
    $user_id = $order->customer_user;
    if( ! empty( $user_id ) && ( $code = get_user_meta($user_id, "gens_referral_id", true) ) != '' ){
        if( $plain_text ){
            echo 'Your referral code is: ' . $code;
        } else {
            echo '<p>Your referral code is: ' .get_home_url() .'?raf='. $code . '</p>';
        }
    }
}
add_action('woocommerce_email_customer_details', 'gens_raf_customer_email', 30, 3 );