My friend is convinced that air conditioning is bad for one's health. is air conditioning bad for you?

No: Your friend is completely wrong. There is no proof or logic behind that claim. As long as the air conditioner is working properly, has adequate ventilation, and adequate drainage, it is fine. If not functioning properly, you can get air pollutants or bacteria/mold in the condensate which can make you sick but not the air from the a/c itself.