Change the email recipient for a specified set of email templates if the intended recipient has an alternative recipient email address set as user meta.

pmpro-change-recipient-for-billing-emails.php

<?php
/**
 * Change the recipient for billing emails to an additional email address.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_change_recipient_for_billing_emails( $user_email, $email ) {

	// Set the user meta key for the additional email address.
	$email_recipient_meta_key = 'additional_email';

	// Set the email templates for which the additional email address should be used.
	// https://www.paidmembershipspro.com/documentation/member-communications/list-of-pmpro-email-templates/
	$email_templates = array( 'invoice', 'billable_invoice', 'billing', 'billing_failure', 'payment_action', 'credit_card_expiring' );

	// Is this an email we should change the recipient for?
	if ( in_array( $email->template, $email_templates, true ) ) {

		// Get the user
		$user = get_user_by( 'email', $user_email );

		// Get the additional email address from the usermeta table.
		$additional_email = get_user_meta( $user->ID, $email_recipient_meta_key, true );

		// Check if additional email is set and is a valid email address.
		if ( $additional_email && is_email( $additional_email ) ) {
			// Return the additional email address.
			return $additional_email;
		}
	}

	return $user_email;
}
add_filter( 'pmpro_email_recipient', 'my_pmpro_change_recipient_for_billing_emails', 10, 2 );