Is there a right owed by a Muslim man who regretted and repented of the sin of fornication with a Christian woman who became pregnant by him and gave birth, especially since there are rights of people that are not nullified by repentance?

The crime of zina (unlawful sexual intercourse) is one of the most heinous crimes, and whoever repents from it with sincere repentance, Allah will accept their repentance. If you have committed this abomination, then repent to your Lord, be sincere in your repentance to Him, and have a good opinion of Him. The woman with whom zina was committed has no right over you, and you must sever your relationship with her and not communicate with her.