A "cultural" marriage is not bigamy under the law, because it has no legal effect. It means nothing more legally than saying she is his girlfriend.Your father has no say over whether your mother can "part ways" or not: she can get a divorce regardless of what he wants. Yes, he will in your state get 50% of her assets, and he will also likely be entitled to spousal suppport from her--but right now, she is supporting him anyway, and what she has acquired during marriage is his as well as hers (he has equal access and rights to marital property as her), so there really is no economic downside to divorce and indeed, there is upside, since it will reduce her from supporting him 100% to simply paying whatever support the court orders and will let her end up with half their assets free of him.