The friend that leaves the bar first pretending to be drunk and unable to walk, let alone drive. He draws the attention of the officer laying-in-wait so the rest of his buddies (who probably *are* drunk) can escape unnoticed.
After your buddy passes the breathalyzer test with a 0.0 blood-alcohol level, the puzzled officer asks, "Sir, I saw you stumble out of that bar like you were under the influence, just asking to be pulled over. Are you nuts?"