I don't think so. I'd bet dollars-to-donuts that they just modified the database info. That would be the best route.
You would just create a page (or modify an existing one) that passes the user_id of the person who clicked the link, along with the thread's id. Then the page being called would check the user_id to see if it is the same as the thread starter, or if it is a moderator.
If it is not not, then stop the script. If it is, then do a query to get the thread subject, and do a simple addition like:
$new_subject = "[RESOLVED]" . $old_subject;
and run an update.
That would only take a few minutes to code.