If her boss told her to do something, and she gave him the finger and refused to do it in front of customers and other employees, that is a bad attitude that would skirt dangerously close to the border of "gross misconduct." Could also be called insubordination, but the employer picks the term.
Loosely, gross misconduct is something you do in the workplace that is so bad you should have known it was wrong to do it, even one single time, without having received warnings. Punching out the employer, stealing money, burning the building down, these are gross misconduct.
If "having a bad attitude" is a general complaint about your girlfriend, not based on some spectacular incident, the unemployment office will ask if she has had any warnings, been told that her attitude needs to change. She will be asked if she knew her job was in danger. Simple misconduct needs to be documented, and the person should be given the chance to change their negative behavior before they are terminated, or the person is more likely to be approved for benefits, even after being fired.
If this has not happened, then she has a pretty good chance of being approved. Remember, an employer can legally fire you just because they are in a bad mood today, but if they do not have enough documentation to show they had a valid misconduct reason for the termination, you may be approved for unemployment benefits.