It is just another damage narcissists do to people around them. And the child continues in life, living a lie, that they never can reveal. Most narcissistic parents are very strict about any child "spilling the beans," "telling family secrets," etc. They give the children a pseudo feeling of responsibility to the parent...and not to laws of the land or morality.
I know a whole family of boys so affected. None of them will admit what their father was, what he did was wrong, or that it affected them. All three are in therapy, have divorced, have trouble at jobs, are irritable people - and never happy. They all share a secret, but are all afraid to face it. Narcissistic parents make their family afraid of stating the truth, for fear of not being loyal to the parent.
The game is complicated, the narcissist makes and changes the rules as he/she goes along, our minds cannot conceive how the fog these children live in clouds thier understanding of what is truth, morality, good living. They just are in a constant state of denial about what they saw/heard/endured in their home growing up.
So, if they cover up for a parent, they are not evil, they are doing as taught, as the Narcissistic parent demanded. It is very sad.