Rat out To inform on or betray someone by revealing their wrongdoing or secrets to authorities or others. He ratted out his accomplices to the police.

Fink on To inform or betray someone by revealing their actions or secrets. He finked on his friends to avoid getting into trouble himself.

Squeal on To inform on someone or betray their trust by providing information about their actions or plans. She squealed on her classmates when they cheated on the exam.

Snitch on To inform authorities or others about someone's wrongdoing or secrets. He snitched on his coworkers for stealing office supplies.

Peach on To inform against or betray someone, typically by providing information to authorities or others that could lead to their trouble or arrest. John decided to peach on his coworker for embezzling funds from the company, as he couldn't stand by and watch the illegal activity continue.

Grass on (British) To report someone's actions or wrongdoing to authorities or others. In the world of organized crime, there's a code of silence, and those who grass on their associates risk severe consequences.