Deine Freundin ist meine Feindin is accepted.

If you're going to be specific about your friend being female, the list of alternatives expects you to specify "female enemy" as well.

"Deine Freundin ist meine Feindin" was accepted. I just entered it. בס״ד

i think gegner is more like opponent (as in sports)

dein Freund and mein Feind are both in the nominative case.

If you saw Ihr Freund -- that's the form used when speaking politely to one or more people; Ihr (always capitalised) is the possessive corresponding to Sie (always capitalised), the polite "you".

i think gegner is more like opponent (as in sports)

The sentence "I have an enemy" accepted "Ich habe einen Gegner".

"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".)