Dob in
Definition 1 of 2
Pro Tip 1/3Trust Impact
Using it can imply betrayal and might affect relationships. 
He didn't realize dobbing his mate in would ruin their friendship.
Pro Tip 2/3Negative Connotation
Often seen as negative; implies disloyalty or tattletaling. 
The classmates were upset for being dobbed in over the prank.
Pro Tip 3/3Colloquial Usage
Primarily used in informal spoken language rather than writing. 
She threatened to dob him in if he copied her homework again.