Parents of children seperated/divorced/not together?

Apart from courts/custody/legal/shared etc parenting duties. When a couple breaks up/seperates etc the parents not married have no legal court stuff for custody Etc. But say the child lives with the mommy but the dad hardly ever calls/picks up the child. Say once every 6 months to a year etc Then all of a sudden the "dad" gets married, has.a job and demands custody shared after 5 years of never being around the Childs life at all? What would happen? Could the mom fight and win? Or would it automatically get worked out as joint? Just a querstion I know ur not lawyers but some may have shared experiences?