I think you really need to relax.
So here’s the thing. A lot of underage kids in college go to parties and drink. You really shouldn’t judge or tell your roommate what to do. Maybe just talk about it ahead of time. Find out if she knows her limits. Agree to what level you will look out for one another. Is someone driving? Will you agree to leave together for safety? What’s the plan here?
Honestly I think you will loo back on this (sooner than later) and laugh at how worked up you are.