Phillip – It is possible to authorize iTunes with multiple Apple IDs, so he can play his old songs as well as his new one. In fact, it would be possible to establish multiple Apple IDs – one for each purchase – so you can sell just the account (and song) that you want.
I doubt that Keith purchased a song with that account, but I will double check the account. If that’s the case then the song wouldn’t have transfered, but I suspect the problem is with emptying the value of the card. I’ll post a comment when I find out.