Overwrite email templates inside Paid Memberships Pro by adding HTML templates to theme directory.

load-html-email-templates-from-theme.php

<?php
/**
 * Overwrite email templates from child theme HTML templates.
 * Create html templates in your child theme under paid-memberships-pro/email/
 * Add this code to your site by following this guide - 
 */
function cg_pmpro_load_from_theme_template( $body, $email ) {

    if ( file_exists( get_stylesheet_directory() . '/paid-memberships-pro/email/' . $email->template . '.html' ) ) {
        $body = file_get_contents( get_stylesheet_directory() . '/paid-memberships-pro/email/' . $email->template . '.html' );
    }

    return $body;
}
add_filter( 'pmpro_email_body', 'cg_pmpro_load_from_theme_template', 15, 2 );

Hello Andrew, thanks for this hook, anyway it cannot works ...
about $body need to get the content instead of the directory, it may be:

if ( file_exists( get_stylesheet_directory() . '/paid-memberships-pro/email/' . $email->template . '.html' ) ) {
        $direct = get_stylesheet_directory() . '/paid-memberships-pro/email/' . $email->template . '.html';
        $body = file_get_contents($direct);

This works. Thanks

Thanks, I've updated the code. Sorry I missed that.