It's not the stomach that's affected by chocolate; it's the nervous system. "Dogs and puppies do not have the ability as we humans do to digest a simple chemical found in chocolate called "Theobromine". When theobromine is ingested by an animal it effects are horrific on the central nervous system and the cardiovascular system. This effect of the chemical has a very harsh effect on the animals body causing severe trauma and often times death. Your dog, if untreated, can quickly go into convulsions, have arrythmia, stop breathing and go into a coma."
Dogs and Chocolate
Dark chocolate is considered baking chocolate, milk chocolate on the other hand, contains much less theobromine. It also concerns me that the dog ingested an artificial sweetener, I'm not sure which kind it was, but some of those have been shown to be deadly as well.
Didn't the vet recommend that you induce vomiting?