Remarriage

My cousin was married to a man 14 years ago. After birth of her child she came to know that the man had his first wife and kids too. After this my cousin never lived together with him. Her child from him is now is twelve years old . From our sources we came to know that man had died three years back. The whereabouts of person are unknown and death certificate could not be procured . My cousin is planning to marry a U S citizen and take her child along. Please guide how can she marry and clear childs status for smooth visa approval.