You may have spider mites, and will need kelthane or neem oil for them. They can give the plant a "dusty" look. I've never had spider mites on petunias, though. Just on marigolds.

You don't easily "see" spider mites. They are so tiny they're barely visible, so if he saw spiders falling out of the plants, they were probably just spiders and not harmful to the plants. I suspect the chemicals were harder on the plants than any insects. If they were healthy looking when you brought them home, why did they need anything sprayed on them? Petunias have a hairy leaf surface and it's not too unusual for them to look dusty.