Why would a child reject a loving parent for an abusive parent?
According to Dr Childress, in his Professional to Professional letter :
Because of the child's unfulfilled attachment bonding to the targeted-rejected parent, the child's emotional pain (grief and sadness) increases when the child is with the targeted parent (because of increased attachment bonding motivations that remain unfulfilled) and decreases when the child is away from the targeted parent (because of lessened attachment bonding motivations, and so a lessened grief response).
The child, however, misinterprets (though the influence of the distorted parenting practices of the narcissistic-borderline parent) this differential rise and fall in emotional pain which varies according to the presence of absence of the targeted-rejected parent, as incorrectly representing that it is something that the targeted parent is doing that is creating this emotional pain, since it increases in the presence of the targeted-rejected parent and decreases when that parent is not present. This false attribution of meaning by the child receives active validation and support from the narcisstic-borderline parent who overtly commiserates with the child over the supposedly "abusive" parenting of the other parent under the guise of being "supportive of the child".
At what age should a child be able to reject a parent? Answer: children never reject parents.