Can Benadryl help itchy dogs with fleas?

In general, no - dogs are notorious for licking at itchy spots on their skin, so you would risk intoxication through ingestion of the spray vehicle. Also, dog skin is very different from human skin (in addition to the dense fur, dog skin is generally thicker and less able to absorb materials through it), so Benadryl spray may have a different effect on your dog than it would on you.