My boundary with my son when he began to drive was that I would add him to my policy and allow him to drive one of my cars but the moment he got any moving violations and especially a DUI he was removed and no longer allowed to drive my vehicles.
He received a DUI and off my policy he went!!!
I know many people from Al-anon who have gotten separate auto policies as well as maintained separate bank accounts. I also know a woman who divorced her husband, put all of their assets into her own name but allowed him to continue living in the home with her but she has no financial liability for his actions.