Have Paid Memberships Pro admin checkout emails come from the customer.

pmpro_email_filter_checkout_from_users.php

/*
	Have admin checkout emails come from the customer.
  
  Add this code to a custom plugin.
*/
function pmpro_email_filter_checkout_from_users($email)
{
	if(strpos($email->template, 'checkout') !== false) {
		//get user info
		$user = get_user_by('login', $email->data['user_login']);
		
		//get from and from name. note that some SMTP servers (e.g. Google Mail)
		//will ignore this and authmatically use the info from the Google account
		$email->from = $user->user_email;
		$email->fromname = $user->display_name;

		//set reply to. even Google will set this
		$email->headers[] = 'Reply-To: ' . $user->display_name . ' <' . $email->user_email . '>';
	}
	
	return $email;
}
add_action('pmpro_email_filter', 'pmpro_email_filter_checkout_from_users');