Yup, your friend is retarded. Maybe the games are the same, but not in that way. It is a series of games. SSBB, look at that game for an example. It is a 3rd installment of the game. More characters, levels whatever. Zelda, has a series, I think. Mario Party, has its series.
Sorry to all you zelda fans, but I am not really into Zelda games that much. I never even played a Zelda game in my life. So I don't know the story. It's like Link is trying to save Zelda or something. I know who Link and Zelda are though. I don't have enough time to play that kind of game. I am probably going to get it for the Wii, because of all the good reviews.
Look at Sony & Microsoft. They are using the same designs aor the systems. Well, not the whole design, parts of it.