I'll do my best. As someone who was rejected by a friend because her fiance didn't like me, I can identify with you.
Some people handle delivering bad news very poorly. You're never going to receive a heartfelt explanation from them because they have bad character. That's the way it is. That's the situation your friend is in.
The only advice I have is to tell her to move on and meet other boys. It's the same hard advice that no one likes to hear when they have feelings for someone.
It's going to take her some time. Time heals all wounds. I would recommend that she stop seeking to contact him because that just drags out the feelings. Life is rough sometimes! I feel for your friend.
As far as the girls going after her. I don't why they would bother because he turned on her. Even so, help her stay strong. She should deal with any threats with confidence. She could even tell them that she will have the police become involved, if they seek to carry out their threats. It's best to give someone something to worry about when threatened. She could tell them that any violence will be met with violence. That's not politically correct. It is what I believe through experience.