There is a huge problem with legal limits. The affects of alcohol are different for everyone. I've known grown men who could down a 40oz of Rye in an evening and still be standing, coherent and functional. I've also known average sized adult men that after 2 beers were slurring and drunk.
Alcohol affects everyone differently. The fact that her homeostasis is 4x the legal limit means her body has probably adapted to the constant presence of alcohol and she wouldn't have any of the symptoms of being drunk.
We need to find a better way to judge someone's ability to operate a vehicle other than the amount of chemicals in their blood.