...On the contrary, why shouldn't he be doing it? Last I checked many people were debating on how to improve the site, and since we have no active programmers to scout out these issues for us, shouldn't any help be appreciated? We can't fix them at the moment, sure, but it's less work for Gerbil whenever he swings by.
Now, I realize I don't have any say in this, but I see no reason for you to shoot mathfreak down automatically when he is clearly trying to help the site. Yes, multiple accounts are against the rules, but in the state we're in currently does it really matter if he isn't using it for serious competition? It's not like whatever he finds has to be made public until they are fixed, and if someone does use bugs to their advantage...you have a banhammer.
If you want the community to see any improvement whatsoever, nay-saying a volunteer outright without any given reason probably isn't the best move.