Question on alcohol posession and consequence?

Recently my friend brit(18 years of age) drove her friend melissa(23 years of age) to the package store in her own(brits) car. 5 minutes after the alcohol was purchased by melissa, cops pull up behind them and demand the 2 to exit the car. The cops explained that they saw melissa leave the package store, and since britney was under age to drink, they said that by law they had to give her a fine, and this was mainly because they couldn't prove who's bottle it was. My point of the story is, is it legal to give the underage (to drink) driver of a car a fine for driving her of age(to drink) friend to get a bottle? the bottle remained closed and in the posession of melissa the whole ride a block away. They also poured the alcohol out. She appealed the ticket and claimed not guilty, any evidence supporting both sides of this story?