Bad breath can be caused by many different things. Of course, it is quite common for it to be caused by the bacteria living in the mouth or on the tongue. However, if bad breath persists despite vigorous mouth hygiene, then it is likely coming from another source.
For example, bad breath associated with acid reflux
from the stomach is fairly common. Additionally, infections of individual teeth can cause bad breath that can be fairly resistant to standard mouth hygiene.
One of the most common causes of bad breath that is often under-appreciated is problems with the sinus or with nasal allergies. Both sinus congestion and nasal allergies cause the lining of the nose and sinuses to become inflamed and congested with mucus and drainage. This mucus then drips down the back of the throat, and frequently leads to bad breath.
Rarely, bad breath can be a sign of a serious underlying medical problem. For example, people who have advanced liver disease often have very bad breath. In diabetics, bad breath can be a sign that the are having trouble with blood sugar control and are getting sick.
If your friend is concerned about their bad breath, they should go to see their primary care doctor