While domestic abuse is distressingly common, it is equally normal for the victim to receive little or no assistance in dealing with an abusive partner or member of the family. Sadly, the "mind your own business" mentality continues to triumph against all logic. It does not seem to strike people that if marriage is not supposed to include abuse, abusive behavior between married people cannot be considered the "relationship". Absurd social assumptions of privacy continue to inadvertently strengthen abuse and stack odds against the victim and allow situations to escalate to levels where the only thing remains is for the chain to break at the weakest link - whether it is a person or a relationship.
But it need not be so. It is possible for socially committed people to influence situations so that abusive people cannot leverage their silence as a consensus against the victim. It is even possible without much risk or effort. What remains is to do it.
How can domestic abuse be prevented by bystanders?
A lot is possible. I'm listing out a few ideas that can be attempted according to the situation, how well you know the victim, and how strongly you feel about the matter.
Voice what is happening
You may not object, but make it clear that you notice what is happening. Abuse thrives in the shadows. For example, one person overruling or suppressing another in a group can easily be acknowledged with something as simple as "five of us want this, 3 want that, and XYZ seemed to want this till ABC stopped her.". In essence, you are doing nothing about the actions of the abuser other than stating them. However, an unfair action being stated reflects badly on the actor and discourages further similar actions out of a wish to not be seen as an unfair person. This is among the safest choices, as you are not required to know any backgrounds of actions beyond what you see, and if it isn't abuse, but has an alternative explanation, you do not end up making any accusations.
Make your disengagement clear
Abuse typically interprets silence of third parties to taste. Your lack of disagreement can be presented as your agreement with the abuser's view even if completely false. It is important to categorically make disagreement clear. While most people hesitate to make a scene, abuse going unchallenged gives it power because it creates the illusion of social sanction. You may not necessarily confront the abuser, but you can easily say something like "I see no harm why s/he shouldn't join us." By providing an alternative perspective, you break the image that "everyone" thinks like that. More so, you never know when others in the group also don't like but remain quiet, and it provides them with a graceful reminder to make their distance known too.
Make a stand
Make your stand clear. You don't have to support the victim, in fact, it is better that you don't in that moment. What you can do is make it clear that the abuser's behavior is unacceptable. "Please don't speak with your wife/husband/child like that in my presence". In this, it is important that you do not side with the victim who could be targeted in retaliation in your absence for things you said. Your confrontation should strictly be between the abuser and you, even if the subject is the victim. This may mean not involving information confided by the victim in particular - which may be seen by the abuser as an attack by the victim to influence you against them.
Understand this. If you are in a position of authority and you don't challenge abuse, you are sanctioning it in the environment. It is absolutely reasonable to use your role and authority to set norms of behavior. "No hitting. No intimidation." "I have asked her for her opinion. If I wanted yours, I'd ask you." Straight, non-negotiable forbidding of inappropriate behavior when you have the authority to do so.
Make it known to the victim that s/he can reach out for help. Cliched as it sounds, offer money if you can. Offer a safe home if you can. This cannot be stressed enough. Money and accommodation are the biggest reasons victims dare not leave abusive relationships. A stash of money comes in Handy for a quick taxi out when shit hits fan. Offer contacts. Discretely collect and share information on inexpensive accommodation if you offering is not possible or otherwise unsuitable. Offer it quietly and in an easy to remember/access manner. Repeat offer periodically, so that it remains in mind as a constant resource that can be trusted not to vanish. You may not be able to offer an option to get out of the abusive environment, but there may be other things that could help. Ask. Ask if there is anything.
The biggest symptom of abuse is a person who withdraws, avoids social contact, feels awkward about answering questions about self and has poor self image. Having company helps. Helps provide a diversity of conversations rather than only abusive ones dominating the victim's interactions, which is how abusive situations narrow and create a perception of isolation and inferiority. Socialization also reduces opportunities of abuse as well as increases the threat of discovery and social disapproval.
Keen an eye, ask friends to occasionally check up on the person. If warranted, provide the victim a simple code that will mean she needs rescued. Something that is not blatant to others. DON'T take it lightly, ever. One of the things that helped me finally decide to risk moving out on my own with a disabled child in tow was something as seemingly unrelated as a day of internet outage resulting in phone calls asking if I was all right since I had vanished online.
Unless you feel capable of taking charge of the victim's well being or offering a substantial part of the assistance needed in recovering, do not do solo rescues. Get police along. The rescue is the beginning. then comes the challenge - of rebuilding life. More difficult to get that assistance after being rescued. Much easier if the police are involved all through. A victim is also less likely to be intimidated into covering up in the presence of the police asking questions of the abuser.
Speak with social workers
They have options, ideas and assistance that could help the person much more effectively. They also have the manpower and diverse competencies to keep an eye, intervene or provide support as needed.
When to call cops rather than be sorry later?
You see unexplained injuries, acute depression to the point of aloofness, inexplicable changes of behaviour that are out of character for the person, hearing loud/angry voices or sounds of objects being thrown/banged/hit/etc, if you have a safety code that gets triggered... if you feel uneasy about the well being of someone in a known abusive situation.
Very likely that it will be a false alarm. Do it anyway. For one, you never know what you prevented by interrupting on time. For another, as someone outside the abuser's control with the power to call cops and get the abuser in serious trouble, you act as a shield. The abuser cannot prevent you from acting in any manner you wish, including reporting possible domestic abuse/crime. It becomes essential that any cops arriving find no trouble. It is as much a deterrent as a response.
If enough people do these simple things, society would be much safer.