He is violating one of nature's laws, and is sowing the seeds of disease.
Once more, remember that the mouth affords no protection to the respiratory organs, and cold air, dust and impurities and germs readily enter by that door.

On the other hand, the nostrils and nasal passages show evidence of the careful design of nature in this respect.

The nostrils are two narrow, tortuous channels, containing numerous bristly hairs which serve the purpose of a filter or sieve to strain the air of its impurities, etc., which are expelled when the breath is exhaled.

Not only do the nostrils serve this important purpose, but they also perform an important function in warming the air inhaled.

The long narrow winding nostrils are filled with warm mucous membrane, which coming in contact with the inhaled air Warms it so that it can do no damage to the delicate organs of the throat, or to the lungs.
No animal, excepting man, sleeps with the mouth open or breathes through the mouth, and in fact it is believed that it is only civilized man who so perverts nature's functions, as the savage and barbarian races almost invariably breathe correctly.