Indeed. I've been thinking about this thread and what I would do. I might help, by securing proper professional help, but I would not necessarily bring this random woman's situation under our roof where our children sleep.
I understand how we might all struggle with this when hospitality is so core to our humanity, but often what feels Nice and what is Right are not the same thing. An adult puts their own household first and their own children before their own interests, virtues, personal safety and reputation.
Someone who puts their children's safety somewhere down the list and indulges in self-congratulatory amateur, unqualified social work bringing other people's possibly violent problems into their home without any idea of how to deal with them is, frankly, halfway between an adult and a child. I have known that attitude to go wrong for the children in a bad way.
It isn't selfless to be selfless with your children's safety. It isn't heroic, and if OH is being the grown-up trying to calm it down, that doesn't make them the bad guy.
Truth be told, if this shit goes tits up who suffers the consequences? Primarily both families' DC. Maybe the adult house guest. Maybe (if he is lucky enough to get a chance to protect his family rather than it being eg petrol through the letterbox) DH as well.