"not... nor" really isn't the most natural way of saying this in English. "neither... nor" is better but wrong, apparently.
It's fixed now apparently
Doesn't "seu amigo" mean his, her, your friend?
What's wrong with "He's not either his friend nor my friend?"
Why is "nem" used twice? Why not: "Ele não é seu amigo nem meu amigo?"
It's used to emphasize. Plus, both "neither" and "nor" are translated as "nem" in Portuguese.
So the sentence can be written with the first "nem"? "Ele não é seu amigo nem meu amigo?" is not wrong then?
Yes, your suggestion is also right.
I agree that the better translation (which should be accepted) is: He is neither your friend nor my friend.