What came of the h being arrested? When did the the incident take place? Was he charged and convicted or is there a criminal case pending?
There is no lack of support from those who are victims of dv, but your friend has to make the effort to access it and be willing to act on the advice she's given.
Her h's threats are hollow, but she's not going to find that out until she removes herself and her dc to a place of safety such as a refuge where all of her needs, and those of her dc, can be addressed.
It may be that she's refusing to coutenance her eldest leaving his current primary school before next summer because the teachng have worked to keep his behaviour under control and it could be she fears that if he acts out in another school this may adversely affect his/her choice of secondary school, but this does not constitute a good reason to keep him in the toxic environment of the family home which has caused him to have behaviour problems.
It may also be that your friend fears that her abilities as a parent may be judged in the confines of a women's refuge and, if this is the case, she needs to understand that any criticism will be constructive and designed to help prevent her eldest ds going completely off the rails if she hasn't got the necessary skills to control, contain, and modify his behaviour.
If there is no professional intervention, so to speak, and presupposing that the council house her direct from the marital home, it could be that she will simply allow her h to move in with her because she remains in thrall to him and/or finds it too difficult to cope with her eldest's behaviour alone.
I'm forming the impression that you haven't been told the whole story as certain actions will have been taken to protect your friend when her h was arrested, and it seems probable she hasn't availed herself of the help/advice that would have been made available to her, either at the at that time or since.
It's to be hoped that the "group sessions" she's attending constitute the Freedom Programme and she will come to realise that, if she expresses herself as wanting to take the first step to freedom, she'll receive all the help she needs to free herself and her dc of her h's malevolent behaviour.