Inform on
Definition 1 of 1
Pro Tip 1/3Negative Connotation
Using 'inform on' often implies betrayal, as it can be seen as disloyal. 
He didn't want to inform on his coworker, fearing the label of a traitor.
Pro Tip 2/3Legal Scenarios
'Inform on' is commonly used in legal or criminal contexts, involving police or authority figures. 
The witness decided to inform on the suspect to help the investigation.
Pro Tip 3/3Whistleblowing
In a workplace, 'inform on' can refer to whistleblowing, which often involves reporting unethical conduct. 
After discovering the fraud, she decided to inform on her company to the authorities.