What is the ruling on repentance from [the sin of] 'zina' (unlawful sexual intercourse), and can Allah forgive the fornicator/adulterer with a sincere intention?

Your friend must initiate sincere repentance with its three conditions: abandoning the sin, feeling remorse, and resolving not to return to it. Allah accepts repentance and replaces evil deeds with good ones, as stated in His Almighty قول: "Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good ones. And ever is Allah Forgiving and Merciful." It is advisable to be cautious of the precursors to indecency, such as mixing (of genders) and private seclusion. If the country he traveled to is a non-Muslim land and there is no necessity or prevailing benefit, then it is obligatory to return to Muslim lands. This child is not attributed to him legally; rather, he is attributed to the husband if the woman is married, or to his mother if she is not married.