What is the ruling on the marriage of a Sri Lankan man who married five years ago with a dowry from ill-gotten gains, knowing that it was so, and what is the ruling on their child?

The aforementioned marriage is valid. As for the money paid as a dowry, there are two scenarios: either it is something that must be returned to its owners, such as stolen property, in which case its equivalent or value must be returned; or it is something that is not Islamically permissible to return to its owners, such as usurious gains. In this case, an equivalent amount must be given out and disposed of by spending it on charitable causes or giving it to the poor, along with repentance to Allah.