Ustadh someone I know fell into Zina and regrets it so much. He believes now he can’t get married because he is az-zānī and he doesn’t want another fornicator. How does such a brother repent?

If he has repented sincerely he is not included in the ayah, because Imam Ibn Baz said the people of knowledge derived from it that a believer does not marry such a one until she repents, and it is the one known for it who is meant. He is not required to disclose what Allah concealed. Repentance suffices and a person keeps himself under Allah's covering.