I have always considered the opposite of friend to be enemy. But in Sydney they think that it is acquaintance.
Here are some extracts from my dictionary:
friend
noun
1 a close friend companion, soul mate, intimate, confidante, confidant, familiar, alter ego, second self, playmate, playfellow, classmate, schoolmate, workmate; ally, associate; sister, brother; best friend, kindred spirit, bosom buddy, bosom friend; informal pal, chum, sidekick, crony, main man, mate, buddy, bud, amigo, compadre, homeboy, homegirl, homie; archaic compeer. antonym enemy.
ORIGIN Old English frēond, of Germanic origin; related to Dutch vriend and German Freund, from an Indo-European root meaning ‘to love,’ shared by free
• a person who is not an enemy or who is on the same side : she was unsure whether he was friend or foe.