To reduce or prevent bad breath:
Brush your teeth after you eat. Keep a toothbrush at work to use after eating. Brush using a fluoride-containing toothpaste at least twice a day, especially after meals. Toothpaste with antibacterial properties has been shown to reduce bad breath odors.
Floss at least once a day. Proper flossing removes food particles and plaque from between your teeth, helping to control bad breath.
Brush your tongue. Your tongue harbors bacteria, so carefully brushing it may help reduce odors. People who have a coated tongue from a significant overgrowth of bacteria (from smoking or dry mouth, for example) may benefit from using a tongue scraper. Or use a toothbrush that has a built-in tongue cleaner.
Avoid dry mouth. To keep your mouth moist, avoid tobacco and drink plenty of water — not coffee, soft drinks or alcohol, which can lead to a drier mouth. Chew gum or suck on candy — preferably sugarless — to stimulate saliva. If you have chronic dry mouth, your dentist or physician may prescribe an artificial saliva preparation or an oral medication that stimulates the flow of saliva.
Adjust your diet. Avoid food and beverages that can cause bad breath. Avoid sticky, sugary foods.
Regularly get a new toothbrush. Change your toothbrush when it becomes frayed, about every three to four months, and choose a soft-bristled toothbrush.
Schedule regular dental checkups. See your dentist on a regular basis — generally once or twice a year — to have your teeth or dentures examined and cleaned.
There could be two possibilities
Does she complain of food getting accumulated in between teeth in certain areas on the mouth?
This may suggest she has cavities in her mouth which need to be attended to immediately.
If cavities are found they need to be filled.
The other being gum problem. Sometimes because of insufficient brushing and oral hygiene measures there is possibility that she has deposits around area between teeth n gums.
Another reason for gum problem could be hormonal changes. Since she is 13, it could be that she is undergoing changes in her body towards maturity. These hormonal changes create inflammation in the gums leading to bad breath and other issues.
She requires to go for a round of cleaning(scaling) and follow oral hygiene methods.
In both cases it is important to bring her to a dental practitioner. The dentist will evaluate the causes and give you suggestions for the treatment.