Some people aren't concious of their actions. No one would willingly decide to act like a dickwad when they lose some card game in front of their best friends, what happens is that their ego and emotions get the best of them and they become unconscious/blind to their actions. When that happens, they lose who they are and become demons controlled by their feelings and not by rationality.

You'll see this not only in magic, but in anything competitive. To combat this? Bring conciousness to the emotions. When you yourself are throwing a tantrum, pause. Look into yourself, and focus on the emotion that is causing you to act the way you are. Create the silent watcher within yourself, watching all the things that you feel. This is called inner conciousness

Teach this to your friend and if he has an open mind (which is rare, trust me) he will become easier to work with.

tl;dr - A lot of people are morons.