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?"
"No sir," he answers, "I'm the designated decoy."