I tend to think it's highly likely that KB took on most (or all) of the upbringing of Zahra, in fact I think that AB was so useless, that KB was still acting as mummy for his needs too...I bet KB did ALL the housework, cooked ALL the meals, paid the bills, did the laundry etc, she probably put his clothes out for him every night before bedtime FFS he likely paid board, but that is about it.
So when he hooked up with EB she stepped into KB's role, especially once they moved to the states, and he sat back uselessly while she treated Zahra with her own special style of mothering.
This is absolutely NO EXCUSE for AB. If he sat back and allowed EB to abuse Zahra, which is quite clear, then he is just as guilty as she is. In fact he is worse, because he should have had more love in his heart for the little girl who he had been with since birth, who he had seen suffer all her life through various illnesses and the losing of her leg which must have been heartbreaking.
But sadly it seems he did not, or is such a pathetic specimen of a man that he didn't have the cahones to do anything!