This idiom is used where I live. This happened this year, where I work and someone went to work for the competition or the enemy.
He is now working for the enemy!
Some define enemy simply as not a friend.
Look up enemy at Dictionary. comearly 13c., "one hateful toward and intent on harming (someone)," from Old French enemi (12c., Modern French ennemi), earlier inimi (9c.) "enemy, adversary, foe; demon, the Devil," from Latin inimicus "an enemy," literally "an unfriend," noun use of adjective meaning "hostile, unfriendly" (source also of Italian nemico, Catalan enamic, Spanish enemigo, Portuguese inimigo), from in- "not" (see in- (1)) + amicus "friend" related to amare "to love" . From c. 1300 in English as "adversary of God, unbeliever, heathen, anti-Christian;" late 14c. as "the Devil;" also late 14c. as "member of an armed, hostile body in a war, feud, etc.;" of the opposing military forces as a whole, from c. 1600. From mid-14c. as an adjective. - The Online Etymology Dictionary
Even Trump has used a similar phrase about the democrats (or if you wish the competitors) as seen in this article: Trump embraces ‘enemy of the people,’ a phrase with a fraught history.