Hello, I'm Dr. Deb.
I'm sorry for this concern for Georgia. If she actually ingested the brownie less than one hour ago, then I would consider inducing vomiting if she's a healthy 12 year old with no pre-existing health issues.
If you have 3% hydrogen peroxide, the dose would be 1 teaspoon per 10 pounds of body weight, mixed with bread and then walk her around. Repeat the dose if necessary.
Not all dogs will vomit when peroxide is given but most will.
If she has health issue where inducing vomiting wouldn't be prudent and/or you don't have access to peroxide, then the following information should be reassuring:
Unfortunately there are no specific antidotes that can be given to reverse the effect of marijuana if a sufficient amount was ingested to cause signs. It takes about 1.5 grams of marijuana per pound of body weight to be fatal so toxic doses aren't usually something we have to worry about.
Signs to monitor which usually start to appear 3 hours after ingestion include: incoordination and stumbling. Many patients become quite lethargic while others may experience hallucinations. Heart rates can go down and pupils can become dilated.
Some dogs will vomit on their own and if Georgia is also quite lethargic, then she could aspirate her vomit and pneumonia may develop (which could be quite serious).
Some dogs actually need to be hospitalized if the signs are severe enough but most dogs will recover within 24 hours if a small amount was ingested.
You basically have to wait it out and make sure that she doesn't hurt herself and is safe until the drug leaves her body.
I hope this helps to reassure you that she'll ultimately be fine especially if the brownie didn't contain a large amount of marijuana and she only ate one. Deb