I'm sorry to hear about your concerns, Polytrip.
She should definitely change the locks immediately.
A restraining order is only possible I would imagine if she herself instigated it. She would need evidence to get one. If there is evidence, she should start collating it. A restraining order probably wouldn't stop a true psychopath anyway, so if grave evidence exists then she might want to move. It is the unfairness of this that makes me think that the American way of home defence has at least some justification.
If you are this worried, would she let you move in with her for a short time to provide the security of a male adult? (Not a sexist comment- the presence of her brother should be an obvious deterrant to him)
Moving out of the neighbood, but staying within visiting distance of friends etc would be a compromise to cause minimum disruption to her and her children's lives.
But to cut herself off from everyone she knows, take her children out of school, and disappear... a self-induced "witness protection programme" that is a massive step that not many people would take unless they KNEW their lives were in danger. Does the risk outweigh the disruption and reduction of her and her children's quality of life?
She could also get in trouble then herself for restricting access to his children.
I mean, while I trust you, I don't know this man and it is possible that you are being overprotective. Maybe he's not as bad as you think? Maybe you could be putting additional strain onto her and her children. It's also possible the other way. Maybe your concerns are more than justified.
It's a really tough situation. I'm no expert of course, I would recommend you or she contact a relevent charity who should be able to advise you better. You can advise her, but you can't make her do anything she doesn't want to... it's her life, after all, she chose to be with this man, and now she is choosing not to see him as as much of a threat as you do. I hope it works out