Legal Question in Civil Litigation in California

I think a small claims court judge would probably find your guest negligent, not only for leaving marijuana brownies out, but for the greater negligence of leaving -chocolate- products, which are toxic to canines, where your dog could get at them.

You should pay the vet. If the visitor caused the problem, then she should reimburse you. If you are each partly responsible, then the visitor should reimburse you in proportion to her share of the fault. But the vet is entitled to his fee regardless of whether or when you get paid by the visitor. Since the dog is yours, the bill is your responsibility.

When you say you "did not know about this", are you referring to the brownies or the marijuana? Chocolate is poisonous to dogs, so even eating regular brownies could have made your dog violently ill or killed him. It is not clear from your question whether the marijuana had anything to do with his illness.

How did the dog get access to the brownies? If you left them where he could reach them, then you may be the one at fault -- especially if his illness was caused by the chocolate and not by the marijuana.

Without more information, that's the best I can do.