though this is a fictional case
but it is very unlikely that the real father on learning that her baby was born dead, did not make inquiries about the dead body of the baby and did not also perform any last rites of the baby
he seemed to have been ok with the fact that her baby was dead
in reality a caring father would obviously ask the hospital to handover the dead body of the baby to him so that he can perform his last rites
it would have been another case when the baby died in the mother's womb itself. but your description does not say that
so considering the above and the fact that the adoptive parents have been taking care of the child like their own daughter, the custody of the child should go to the adoptive parents, because ultimately what the Courts look at is 'the welfare of the child'
the welfare of the child would not be secured by a father who did not even bother to claim the baby's dead body upon learning that she was born dead