Weird, anything over 18 is an adult, and i find it ridiculus that youre making a big deal that she drinks at 20!
Anyway not all 20 year old are bad!!
Nor is she making a big deal over the fact that she's 20 and drinks.
In fact, she stated several times that she doesn't care if the gal drinks -- and said she'll offer a ride home if the roomie's too drunk to drive.
Here in American you can get ticketed if someone in your house is drinking... regardless if you provided the alcohol or not... you can be guilty by association.
She's not making a big deal out of anything, she just doesn't want to get a ticket. And I think that is fair.
She came asking for advice about how to deal with a roommate situation, not whether it's legal or right for her roommate to drink or not.
Good grief people!