If it's anything like GFWL IDs or multiplayer keys or Steam codes, that code may be one use only and it's already locked to someone elses -the person who owned the game before- account.
He`s got a account so stats should be recorded for this account. He probably only needs a code to activate it. Try to email the statsside team first and maybe pm Gstaff if nobody will answer from the statsside.
Or... if you bought the game used from someone directly, contact them and ask them to unregister the account from the stats site. This could take less time than contacting the stats-site and getting a response. Good luck.