Well, one must asked, Does the woma have a history of getting into abusive relationships. ANd does the man have a history of abuse. Chances are to both those questions are yes.
If the guy is abusive, they tend to either be very controlling due to insecurities, and lack of being about express their fellings and lack of communication skills, often times it could be a learned behavior (from seeing Dad hit mom).
To help a woman out of the situation, one needs to convince her to go to a shelter, and NOT TO CONTACT THE BOYFRIEND! Under no circumstances, if kids are involved she should take the kids. Meanwhile as a woman friend, you should help build her self-esteem up to the point where SHE HAS TO SEE THAT SHE CAN DO BETTER THAN HIM.
Also convince her that a man does not beat the woman that he loves, under no circumstances, and that it is not her fault. These are two common reasons woman will give as "explanations".
All ina ll that is what you can do. As there is a saying, "If you are looking for a helping hand, look at the end of your arm." The woman has to realize that abusive relationships are not good for her mentally, physically, and emotionally, and especially if she has children.