Well typically it’s halitosis (bacteria chillin’ like villains in yer mouth). However since she will be brushing her teeth and chewing gum here is a good list of the shiznit that could also be a causing it.
Medical Causes of Bad Breath
1. Sinus infections & abnormal sinus anatomies, post nasal drainage (drips)
2. Tonsilar infections or tonsiloliths
3. Lung diseases
4. Kidney diseases
5. Liver diseases
6. Blood disorders
7. Diabetes
8. Gallbladder dysfunction
9. Menstruation
10. Carcinomas
11. Certain foods
Dental Causes of Bad Breath
1. Extensive dental decay
2. Periodontal (gum) disease
3. Oral infections or abscesses
4. Oral cancers
5. Xerostomia (dry mouth condition). Many medications can contribute to a condition of xerostomia.
6. Oral conditions resulting from post-nasal drips or discharges
7. Allergy conditions
8. A proliferation of specific types of gram (-) anaerobic bacteria.
Beyond these you might consider trying to “cleanse” out your organs. Liver, kidney, colon etc.. because doing that will improve your digestion and your body will better be able to process all the crap you throw at it without making you blow out putrid smelling liquid. There are tons of cleanses at www.curezone.com