Bad breath like fermenting

My girlfriend deals with bad breath every now and then. It smells... intense. Usually it's around her period but not every month. Other times it doesn't seem related with that. It kind of smells fermented or like someone (sorry to say it like this) drooled in their sleep, let it dry and coat it like this 5 more times. Sometimes the smell is very present in the entire room, not just near her but I'm certain it comes from her mouth.

Her dentist says everything is fine, obviously it's not. So I'm guessing it could be something about her stomach/digestion. She also had er tonsils removed when she was a child due to inflammation - if that's relevant.

Anybody know how MM could deal with this?