If A person marries and becomes a citizen can anything be done to revoke his citizenship if it is later found out it was all committed by fraud ?
Since he is already married in his own country he lied on his marriage application and INS ?

That would invalidate his marriage, and that would have invalidated all their paperwork on the application... and sworn statements. I would mopst definitely turn them in, and give as much info as you know about them to aid the investigation.

Only the first marriage is held as valid unless there is eithe ra divorce, anullment or death of that partner. Any subsequent marriage is not legal or recognised.