allow referrals for registered customers only

functions.php

<?php

add_filter('gens_raf_generate_coupon', 'dont_generate',10,2);

function dont_generate($generate_coupon,$order_id){
  $order = new WC_Order($order_id);
  $user = get_user_by('email', $order->billing_email);

  if(!isset($user->ID)){
     return false;
  }
  return true;
}