His friend broke his confidence and went ahead and told the restaurant people what was afoot so they'd be prepared for it.
When the day arrived things went according to plan. He took out the plastic worm and put it in the salad..then he made a fuss saying aloud "HEY! What's with the worm in my salad!"
At that moment the kitchen doors sprang open and two big dudes in chef uniforms came out, grabbed him and took him back into the kitchen. I am pretty sure the onlookers in the restaurant who may have considered criticizing the food quickly changed their mind. Meanwhile everybody in the kitchen had a good laugh.
Customer: WAITER! WHAT’S THIS FLY DOING IN MY SOUP?
Waiter [Peering into bowl]: The Backstroke, I believe, sir................