Even the most beautiful of Girls can lose a man with a stinking attitude, the novelty of looks wears off quicker than people think. Most men in reality want a beautiful Girl that will also feed their ego and be a rock solid support, those that throw tantrums all the time and are a bit ditzy won't last long.
Yes that's true, but as said earlier, he should end the relationship then, not cheat. Clearly she wasn't such a bad girlfriend that he was prepared to leave her. And *if* the reports are to believed, he's devastated and wants Kelly back.
If you don't want to be with someone, don't be with them, but tell them. There's no excuse for cheating.