I can relate to your story on so many levels, Mr. Concerned. I have a 16 year old stepson who behaves in a very similar manner to your ex girlfriend's son. He strangled my wife and expressed zero remorse. He is highly resistant to therapy. He has anxiety and depression but won't take his medications - he is only functional when medicated. Otherwise he is a raging, moody monster. We haven't seen this child since April when at my wife's and my insistence, following multiple ER visits due to his violence in the home (he lives with his father and we have every other week visitation, which has not been happening), he was placed in a partial hospitalization program. His father is a champion enabler and if stepson doesn't want to do something, his father will make sure he doesn't have to do it. This goes for homework, chores, everything. My wife and I have no influence and no say because father will simply override my wife's wishes, decisions etc and give in to stepson.
I have struggled with keeping my nose out of this situation in my own family. I am very sorry that the stress of having such a difficult child has temporarily derailed your relationship with his mom. Hopefully in time you can resume the relationship. I am still working on boundaries. My wife and I are in agreement that this stepson should not be in our home with his current condition raging unmedicated and unameliorated - not that it matters because he refuses to take her calls, let alone see her. He has cut off my wife's entire family. And I am glad for that. He is a danger to himself and others and I see nothing but heartache ahead for him and anyone he comes in contact with, barring a miracle which I don't see coming at this point.
Your ex's son has indeed committed assault by putting his hands on her. I hope she finds the strength to give him the consequences he so desperately needs. Once he is 18 and a legal adult, he will find the courts have no sympathy for people like him,