Friend's Abusive Father Going Too Far

Hi - I have known a female friend of mine for roughly a year and a half. She has mentioned to me before that her family situation is not good. I have audibly witnessed this myself, being on the phone with her when her father has "gone off the deep end", on more than one occasion. I am posting this to let you all know the situation as I understand it, so that you might be able to give me some insight as to what options there are available; in regard to making this stop.
Basically this family consists of the father, the mother, the oldest child; a boy, my friend; who is seventeen (and of legal age to move out, etc. in our state), and their little sister, who is, I believe, five years old.
The father, coming from a home where he was beaten up and verbally abused regularly by his older brothers and his own father, is an alcoholic, and has been known to have episodes of extremely hard drug use. He works only when he needs money, and is the type of person who has the friendship of a lot of low-life individuals who enable him. They also get him out of most trouble he finds himself in, as a result of his increasingly dangerous actions.
I have been informed, in a very serious and emotional manner, that this man used to physically beat his oldest child - my friend's brother - and verbally abuse him. She cited at least one incident where her father "head butted" her brother in the face, knocking him out.
He has roughed her mother up considerably, leaving severe bruises in places not easily noticeable when she is clothed, such as her arms, and sometimes the scalp, which, naturally is hidden by her dark hair. My friend has stated that she has never had any reason to believe that her father has actually "beaten up" her mother, but that she knows her mother doesn't tell her everything, in regard to what all has transpired between her and her husband.
This woman, from what I am gathering, is the type who would rather turn the other cheek, than see her husband hurt her family, which he has apparently done before. He has run members of her side of the family off the road, and, according to my friend, has physically attacked and beaten several of these individuals.
All this is confirmed by more than one source, by the way. My friend has informed me also that every time they do get up the nerve to have her father arrested, or put in rehab, a childhood friend of his, who happens to be a so-called preacher (which anyone can be in the south, mind you) continually bails him out of jail, or uses his influence to have him released somehow. He claims it is so the man can "get help", or even so that the preacher can help him, but he does nothing. He bails my friend's father out of jail or gets him out of rehab, knowing full well that this man intends to get even with his family for having him put in, in the first place. Because he is "a preacher", no legal action is ever brought against him in connection with this.
Once, when my friend's father was locked in jail, to "get back" at his wife for calling the police, he made a call or passed a message to someone, and had six or seven of his lowlife friends beat up his wife's cousin as he was leaving a bar alone. They left this man in a mild coma, lying in a ditch. Again, nothing could be proven at the time, so my friend's father went unpunished.
My friend's mother, and my friend herself, are both entirely too forgiving, and refuse to do anything else to have this man locked away or forced to get help. They are partially afraid of what he'll do, as they have seen first hand that he always manages to get out somehow. They also still love him, despite the way he treats them. He has made verbal threats to them; saying that if it "comes to it", he will kill them and himself, leaving only the youngest child, the little girl, alive. This man is a dangerous, abusive drug addict and alcoholic, who has somehow managed to fly under the radar, abusing his family and committing various crimes; both petty and quite serious, for several years.
There has got to be some way to get him locked away somewhere where he can't get messages out, and can't have his scumbag friends bail, beg, borrow, or bribe his way out. He needs to go away for a long time, and be subjected to intense behavioral modification therapy, before he does something irreversable. This is a man who has beaten people, trafficked in illegal drugs, verbally abused (and is displaying fringe signs of physical abuse toward) his family, and has made threats of killing both himself, and others.
He has gotten himself in deep trouble with inmates in the main correctional facility for people in his category of offenses, and fears that he will be the victim of an attempted murder, should he ever face going back into incarceration. In regard to this, he has stated flatly, that if it becomes clear he will go back to that facility, he will "do whatever it takes" to see that he doesn't go back. He has implied heavily that he is prepared to kill himself, and to take his family, and any police or correctional officers with him. Please tell me what can be done to get this man away from my friend and her family, without the possibility of retaliation from him or his friends. Thank you all so much for reading this, and for any help you can provide, in the form of answers, suggestions, or just plain support. I sincerely applaud you all for having the courage to discuss these topics in an open and adult manner, and also for caring that things like this go on in the world; it seems that fewer and fewer people do these days. I look forward to reading your comments - Thank you again, and best wishes!

I think it is very commendable that you wish to help your friend, rather than turn the other cheek and act like you don't want to be involved.

I'm not sure about your country, but here in Australia we have a Child Abuse Line that can be called and you can give them details of your suspicions and they will document the examples you give them. Your call remains anonymous. The details of your call are passed through to Child Protective Agency in their area and the call is followed up.

I'd be pretty sure that you can do down a similiar path if you are not in Australia.

There's a defenceless 5 year old girl who is going to be very adversely affected by her alcoholic father and if her mother is not strong enough to remove herself and her kids from this abusive man then perhaps it is time the authorities were called to see what they can do.

Best of luck