She failed a roadside breath test IIRC.
She was not given a roadside breath test, there is absolutely nothing to say she was even still drunk when arrested.
I think she just had the hump about them driving her mad in her car and it all escalated once at the police station.
You just need to put this down as not being one of plods finest hours.
Also it wasnt just a black eye, she needed stitches.