Hello and welcome,
Bad odour from mouth is usually caused due to poor oral hygiene
Deposition of food debris and plaque can result in formation of hard calcific deposits of plaque known as calculus.
This can lead into gingivitis
and which can progress into periodontal infection
I would advice you to get deep scaling done for your girlfriend.
Rinse mouth after every meals.
mouthrinse to gargle.
Use soft interdental toothbrush and dental floss to clean teeth and tongue scrapers to clean tongue.
Maintain oral hygiene well.