Ans.56: First, she is not his wife. It is known that Islaam wipes out what was before it but it is upon this couple to stop living together and not spent time together. After that, there is nothing wrong for them to perform a Nikah (marriage) with a contract, walee and witnesses. If they had children before, then they are not his children, they are only her children even if he marries her after this, he becomes her husband only.