Feeding a dog mushrooms is acceptable if the mushrooms were purchased at a grocery store and are safe for human consumption. Nothing more than a small amount should be fed to the dog, as the risk of stomach issues like aches and vomiting increase the more mushrooms it is fed.
Wild mushrooms are never acceptable to feed to a dog due to the risks of liver and kidney problems. More serious complications such as neurological issues and even death are possible. When feeding a dog store-bought mushrooms, it is best to avoid seasoning the mushrooms in any way. An inordinate amount of salt can be dangerous to dogs.