Make auto-input fields read only. For use with Event Espresso 4 + the WP Users integration add-on

functions.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function ee_readonly_autofilled_inputs(){
    wp_add_inline_script( 
        'jquery-validate',
        'jQuery(document).ready(function($){
            $.extend($.expr[":"],{
                textboxFilled: function(el){
                    return $(el).val() !== "";
                }
            });
            $("#spco-attendee_information-dv .ee-reg-form-attendee-dv:first :text:textboxFilled").prop("readonly", true);
        });'
    );
}
add_action( 'wp_enqueue_scripts', 'ee_readonly_autofilled_inputs', 50 );