I'd sit her down & talk to her.
Maybe give her til 1 or 2 am & let her know you'd like to pick her up. Tell her it's not her you don't trust, it's the fact that people of ALL ages get silly when they drink, & you don't want to risk something happening to her, or her bf.
If she stays sober, she'll hopefully be tired by then anyway.
I personally am OK with maybe a drink on a special occasion under age, but not 'drinking'.
Just explainhow you feel. Let her know that 'yes it is more than likely everything will be fine, but there is still a chance a fight may break out or something & she may get hurt. You just don't want her to get hurt.
People turn on others for no reason when drinking.