Abusive possessive fiancé

My cousin is engaged to her boyfriend she has been with for a little bit over a year now. She's 24 and 2 months pregnant and her fianc was in the Marines training for a couple of months and they were planning on getting married sometime this month. After he came back, they started arguing more and on Tuesday he slapped her in the face because of something she said that he thought she was trying to hurt him personally. She knows that he's going around trying to talk to and meet other women and has confronted him about that too. He becomes paranoid all the time trying to control her and seeing who she is always with. He has trust issues. I'm afraid of how her situation is going to be. She doesn't have a stable job right now. I just don't know how I can help her.

Also, the problem is no one except her parents, siblings and I know about her being pregnant. And she plans to keep it that way until she is married. She's not currently living with him right now. She's living with her parents. She doesn't want to call the police because she's afraid her parents will worry. Her fiancs mother knows about it but cannot really do much. I personally think he has mental issues. He's seems very abusive and controlling. He can become angry easily.