What is the ruling of Islamic law on abortion if a woman is pregnant from adultery and is in her waiting period, and she does not wish to have an abortion? And does the man's sin increase if he abandons her?

Abortion is forbidden and is not permissible except out of necessity and in specific cases. Pregnancy resulting from adultery is not a justification for abortion, because it is the destruction of offspring, corruption on earth, and an assault on a created soul. This does not remove the crime of adultery, but rather adds another crime to it. Those who commit this major sin must repent to Allah, and the brother is not obligated to marry the woman.