"You are my real friend."
Translation:Jesteś moją prawdziwą przyjaciółką.
To be a bit more specific, as a friend it is totally archaic, to the point most poles wouldn't make the connection. As a Scouts commander it is not archaic at all, but that is not the meaning LICA asked for, obviously. ;-)
Rare, much rarer than „przyjaciel” but not really archaic. I'm not even quarter century old and I would have no problems recognising it. A song from Toy Story uses it, for example.
Well, a song is not really indicative, since you might use a lot of "weird" words in order to fit the rhythm and rime, but checking NKJP it seems you and mihxal are right, through I can't remember meeting the word "in the wild", other than some 50+ years old books and stuff stylized for old – guess it just isn't used by my friends/family.
If przyjaciółka can in some sense be rendered as girlfriend, then it is an ambiguous answer to say it can be only as friend.
If you call your girlfriend "przyjaciółka", I guess you also have "it's complicated" as your relationship status on Facebook. Yeah, I guess someone may say that, just like someone can call his girlfriend "friend" in English, but that's absolutely not a translation.
Would I be right in thinking that you were actually using "impersonal you" here, and were in fact referring to a hypothetical Facebook account, not the poster's?