DeaD man WalkiN: some people simply don't know the rules. Don't get angry; refer them to read the rules, and to stop playing if they don't like them.

Aside from that, I can understand that the doubling cube can be an extremely annoying thing, especially if you don't understand it - your opponent roles a lucky 6-6 and turns the game around, and then has the audacity to double... but that is Backgammon. [this is an explanation, not an excuse!]
He needs to learn to use the cube. And if he cannot lose, he should go play with dolls instead.