A.W.
Hey there,
You are a good friend for seeking advice for her.
If there is fear anywhere in her - the relationship is abusive. You should never fear your partner. Even in an argument, I have never in my life been afraid of something my husband will say or do. Ever.
If she does want to leave, but is scared about what he will do, she needs to seek legal protection from him before she does anything else. I would suggest filing a restraining order against him and then filing for divorce, preferably as close to one another as possible. Restraining order FIRST - because once he is served divorce papers, she may need the restraining order against him if he gets angry.
Which state is she in? Have her look into her home state's Department of Children and Family Services - there are a lot of resources there for her and the kids. All she needs to do is speak with a case worker and explain her situation. They will help her and protect them from him should he become more abusive.
Best advice I'd give to her now? - Fake it. She should NOT just walk in one day and say "Its over" because that is an open invitation for a huge blow up. While they're living under the same roof, and ESPECIALLY if she thinks the man may be bipolar or something - fake it. Don't have any long conversations about any of this. Just go through the motions, and tell her to work behind the scenes to get out of there safely with the kids.
If it gets really bad she should arrange to stay at a friends house with the kids and go to the police to press charges. Depending on which state she lives in, there are a number os charges she could press against him legally that would stick long enough to have a restraining order put through quickly and to have her possibly assigned to an officer that can stay with her and the kids.
Encourage her not to go anywhere without the kids. If it were me, I'd personally open his skull with a hammer if he came near my boys. But thats just me being pregnant, hormonal and a little too mama bear.
There you have it. DCFS, restraining order, divorce papers. And then some much needed family counseling for her and the kids.