diablo cd key

Hey, yeah I was wonder if you guys know if you need a diablo 2 reg cd key that no one uses to play LOD....cause my friend has a reg diablo cd and he said I could use his cd key for reg and just buy my own LOD and it'll work but my experiances with diablo cd keys tell me wrong. Last time I tried that I couldnt play when my friend played so if anyone knows it would be great to get a response thnx all