In some situations, over a period of years the child has bonded with the dad and the relationship is loving. Resolving a mistaken or fraudulent paternity matter in that circumstance is difficult. The truth usually only comes to light because the parents are splitting up and the mother is either looking to hurt her ex or is bargaining for custody. The man doesn't necessarily turn his back on the child as much as he is running for cover after being bombarded with devaststing testimony by his ex. In time the wounds on both sides (father and child)may heal, after the lawyers are gone.
This is one of those topics that, even though it hasn't happened to me, the subject still raises my blood pressure. IMHO, anyone who would let a woman who commits this kind a fraud off with no consequences is almost as bad as the person who committed the fraud. As was mentioned in another post, the policy of allowing this to happen is legally establishing that two wrongs make a right.