Just to be on the safe side I would call the vet and see if they will give you prescription food for them for the next week or two, something that will promote healing of the lining of the intestines and stomach.
My dog Ed got into valentines chocolate when he was about one and ever since he had MAJOR stomach issues... We took him to the vet immediately after we saw he ate the chocolate, and he did the prescription food and had medication. For a few days afterwords he had bloody diarrhea and vomited on his own. It was horrible and he had issues with puking ever since he ate the chocolate (he would puke everyday withing an hour of eating)... we ended up switching him to a food that was for sensitive stomaches and he did a lot better on that food.
The chocolate didn't kill him (cancer killed him), but it did cause permanent damage to his stomach even though he was taken directly to the vet, the damage had already been done.
That being said... my dog Gunner ate some Andes Mints back in December and was fine. Chocolate effects every dog differently.... even 2 dogs that come from the same litter (Gunner & Ed were litter mates).