I'm thinking of letting a girl believe regular brownies were "weed brownies" and letting her make a fool of herself all night in front of our friends
That's not nice at all. Why are you thinking of pranking the girl?
It will be a good night for laughs.
You should tell the someone what the food they are eating actually contains. Lying about the contents of a food will only make the person rightfully upset with you afterwards.
It was a joke.
You are being mean trying to make someone out to be a fool. Just let her know they are normal and stop lying to her.