What happens if custody is granted to the mother but the father takes the child and won't give him or her back to the mother.

In the other thread, which a moderator has apparently deleted, you indicated that your BF is the father of one child, an 11 month old baby. The mother of this baby has two other children, one older and one younger (new-born, evidently).

Is the new-born the baby which you are asking about? Assuming your BF is the father, he can sue the mother for custody. Until he gets a court order for custody, either parent has the right to keep the baby.