My name is***** and I am sorry to hear that your dog is indulging in your guest's treats. In terms of the toxicity of the treat, I am much more concerned about the chocolate than the marijuana. About how large was the treat? Was it a brownie or a dark chocolate cube? How is your dog acting now? Any change in behavior? Any vomiting or diarrhea?
Chocolate is toxic to dogs because it acts like a stimulant. The dogs will get high heart rate, high blood pressure, hyperactivity, vomiting and diarrhea. Mild chocolate and brownies are usually not very toxic unless the dogs eats a very large quantity, but dark chocolate can be much more toxic.
Marijuana in dogs is a downer. They will become lethargic, weak, ataxic (loss of balance), small pupils, drooling, dribbling urine and very slow to respond. At very high doses, they can become hypothermic, have seizures and lose consciousness. Most of the time dogs are eating the actual dried plant or treats with the plant baked into it. This usually means that they are not getting nearly enough to become life threatening but they will become "stoned." I recommend wrapping them in a blanked and placing them in a quiet dark room to sleep it off. Most lose their appetites (different from humans) and all will not drink while intoxicated.
I hope this answer helps you. Feel free to contact me if you have any further questions.
Kansas State University
College of Veterinary Medicine Class of 2001