@turquoise-- It's not a good idea to jump to conclusions, because there are many other causes of mood swings too. It can be a health condition or a psychological condition. How old is your friend? He could just be going through a hormonal change.
I have severe mood swings a week before my menstruation. I become a monster and pick fights with everyone around me. I return to normal after a week. Hormones play a very big part in our mood and emotions, more so than we realize.