You shouldn't call the cops on a friend.
John and Jim are leaving a party and Jim is very drunk and wants to drive himself home.
John wants to keep Jim from hurting himself.
John convinces Jim to ride home in John's car instead, saying it isn't safe for Jim to drive.
John calls the cops as Jim is getting in the car and tells them there's a drunk driver in the area.
Jim appreciates John looking out for Jim's best interest and vows to return the favor sometime.
The cops stop Jim and take him into custody, and Jim is furious at John when he finds out it was him who called them.