A Friend of the Enemy
People who pretend to like you, but they're actually working with the enemy behind your back. They're like a betrayal in disguise.
My coworker is a friend of the enemy. He acts like he's my best friend. But he's been taking my ideas and giving them to my boss.
My friend is a friend of the enemy. She acts like she's on my side. But she's been giving me the worst advice.
My neighbor is a friend of the enemy. She's been talking to my mom. Now she knows all my secrets.