It's entirely possible that Trudy already owned a Howie coat completely independent of Halloween, and threw it on as a costume when she had decided to go to the dance after all. Of course, she isn't here to dance — she's here for revenge. It shouldn't be hard to find Tony, considering he's impossibly tall, but it's catching him when he isn't completely surrounded by people that's proving to be the trouble. She wants there to be no witnesses to her mischief.