What is incumbent upon a man who committed adultery, and as a result of his adultery, a son was born who was then killed by the adulteress in his presence, and then they both repented of this act?

What the man and woman did requires sincere repentance with its conditions; if fulfilled, Allah will accept it.

The woman who killed her child must pay the blood money (diyah):

If she killed her child after birth: the full diyah is obligatory.

If she aborted it stillborn: one-tenth of a woman's diyah is obligatory.

The woman does not inherit anything from the diyah of this deceased.