Rat on
Definition 1 of 1
Pro Tip 1/3Negative Connotation
Using 'rat on' suggests disapproval of the informant, as it's often seen as a betrayal. 
Even with the evidence piling up, she refused to rat on her friend.
Pro Tip 2/3Slang Usage
It's casual slang, not suitable in formal contexts like business meetings or academic writing. 
I couldn't believe he would actually rat on me to the teacher.
Pro Tip 3/3Criminal Context
Often used in the context of crime, where someone reports illegal activities to the authorities. 
After being caught, the thief decided to rat on his accomplices.