A serial code can only be used once - for one account only. Therefore, you'd have to be using her account or the account that you originally redeemed it on. It's just the way things work.
If you need assistance with recovering the account, please contact a game advisor: http://eahelp.ea.com/en/ . For help with navigating the site, see this: http://answers.ea.com/t5/Beta-Archive/How-to-contact-a-Game-Advisor-using-the-new-EA-Help-page/m-p/2161281
