Lineage of child

One of my friend had sex with his real brother's wife and she got pregnant and delivered a child. And even his brother don't know about this and he has accepted that child as his own. Now he is very ashamed on his act but seeking forgiveness from Allah. Kindly suggest what can be done for kaffarah?

بسم الله الرحمن الرحيم

The child shall belong to the husband as the woman is in his nikah. Now the kaffarah of adultery is to do true and genuine taubah and seek forgiveness from Allah.

Allah knows Best!

Darul Ifta,