The grade of gas dosent have anything to do with gas mileage. The octane level in gas is to prevent detonation (knock) from occuring. Knock occurs when the spark in the cylinder on the upstroke (when the piston is moving toward the spark plug) prematurly detonates the fuel in the cylinder...
Regardless if she is your best friend or not, she is drunk and shouldnt be driving. Keep in touch with her, and either go pick her up yourself, or have someone sober go get her. Its not worth it to leave her life to chance.