Depends on the mushrooms.
If they're pixie-caps, then Vinny's answer might be spot on...
Otherwise, there are hundreds of kinds, most inedible to humans, some edible, some poisonous.
Things edible to humans may not be so to dogs -- for example onions are quite toxic to dogs.
If your dog is still right way up after a few days, it's probably not a poisonous mushroom.
If you can train your dog to find edible mushrooms, can I borrow it? My dog said "so what, it's a mushroom" and went on chasing sticks.