Two-faced

• For example, “I thought she was my friend, but she turned out to be two-faced and spread rumors about me.”
• In a workplace setting, someone might say, “Watch out for that coworker, they’re known for being two-faced.”
• A person reflecting on a past relationship might say, “I should have realized he was two-faced when he started talking badly about his friends behind their backs.”

Fool around

• For example, a friend might say, “I heard they were fooling around behind their partner’s back.”
• Someone might confess, “I made a mistake and fooled around with someone at a party.”
• A person discussing infidelity might say, “Fooling around is a breach of trust in a relationship.”