Battlefield 2142 CD-Key help

I bought a Battlefield 2142 game from this dude who used it before, and now I want to go online but it says "invalid CD-Key"..... Why would that be? Is it because the other guy used it already? If so, is there any way I can get around this?

Well, I got it to acknoledge that my CD-Key is correct, but here's the bad news. It says that the cd-key is already registered with the other guys account and I have to logon with his account, problem his, he doesn't remember the login or password, so is there something I can use to find out the account and password registered with the cd-key?