Good point, Pat Putnam . My story is not intended to be medical advice. (Also, this happened more than 15 years ago, and I am now with a different vet.)
In similar phone calls I have made to vets re dogs eating bad things — all my calls involved rasin-containing food — the vet has recommended inducing vomiting with hydrogen peroxide.
I was surprised the vet did not have the dog come in or suggest inducing vomiting, but then I am not a vet. From her comment about the substances canceling each other, I surmise she thought the sedative properties of the pot would dampen the stimulant properties of the chocolate and vice versa. And, the vet did say to watch the dog and bring him in if he started behaving strangely.
I guess she was right in this particular case, because the dog came through the incident unharmed. It was a large dog, as I discovered upon our accidental meeting, and I was not privy to the actual dose.