How does a narcissistic parent react or feel when they lose one of their children to suicide?

It depends. If, at the time of death, the child was functioning as a source of narcissistic supply to the narcissist - then the narcissist will mourn and grieve. Otherwise, he is likely to remain indifferent (though he may pretend otherwise in order to maintain his standing in society and his relationship with the other parent).

I'm kind of curious of the reverse of this question. My abuser's mother comitted suicide right after an argument with him. He seemed distant and indifferent at first, but then it hit him and he left me the day before my birthday for a while so he could drink/do drugs/date a different girl in peace. Then he came back to me and picked up the abuse. I've since left him, but I just thought the question peculiarly similar to my own situation.