The additional code needs to be inserted after submitted_email is declared, at about line 90.
I pasted it after this bit of code:
if (check_email($submitted_email) && $send_from_users_email === false) {
$from = $reply_to = $_SESSION['form'][$mail_from_name].' <'.$submitted_email.'>';
} else {
$from = '<'.$email.'>';
$reply_to = check_email($submitted_email) ? '<'.$submitted_email.'>' : $from;
}