Is it permissible in Islam to adopt an illegitimate child, and is my friend’s reaction to it correct?

Asalamalikum. One of my muslim brother’s sister-in-law has adopted a male child. The child is illegitimate. The Hindu mother gave birth to the child and left it in hospital. Through hospital child has been adopted and background of the child had been revealed by the hospital before the adoption took place. No one knows about child’s father. My friend has a strong objection to this saying adopting illegitimate child is a great sin. Hence he avoid visting his father-in-law and mother-in-law who is staying with their daughter who adopted the child. He also restricts his wife from visting her parents. Kindly give the reply as per Islam ruling whether such adoption is permitted in Islam and whether my friend’s reaction is correct according to Islam. Insha Allah also include us in your Dua.