How to win child custody after a Divorce?

My cousin married and had a child in 2018. In 2019, the child was born. Her husband abandoned the house in 2020 and has not returned or paid any maintenance since. The child has turned five years old. We want custody of the child. What happens to the custody of the child if my cousin divorces her husband? Will the husband get visitation or have say in major decisions about the child's life? Can my cousin have a new child through IVF without the consent of Husband ? An unknown donor is an IVF participant. Is the husband entitled to any say in this matter if he is not legally divorced? Will this affect my cousin's ability to obtain custody of the first child if divorced? Is it possible for my cousin to adopt another child? If my cousin chooses IVF, can the husband take custody of the first child? The court may view her decision as irresponsible or as evidence that she is not putting the child's best interests first