"Your friend is my enemy."
Translation:Dein Freund ist mein Feind.
"to be" is not a transitive verb that takes a direct object.
You can't say that the enemy "is being been" by your friend.
"to be" is a copula - a linking verb that links a subject to a predicate that says something about the subject.
Such predicates are (almost always) in the nominative case in English. (And, traditionally, in English -- conservative speakers will say "it is I" and not "it's me".)
Isn't "Eure Freund" the correct way of saying "Your friend" adressing two or more people?
Freund is masculine, but eure is the form used before a feminine or a plural noun. (Mnemonic: *die ends in e.)
It would have to be euer Freund, with no ending at all after the r, as in ein Freund.