Sorry for long post!
As others have said, you can't make her leave but there are some things you can do:
Do bring up the subject (obvs not when he's around!) - it might feel uncomfortable but keeping it as a shameful subject is more dangerous
Try not to criticise her partner or the relationship. Instead, focus on the abuse and her safety. One of the things that abuser's do is to keep all the attention and energy focused on them so you need to undermine this by encouraging her to consider her own needs and feelings.
Supporting a friend in this way is a huge challenge. You don’t want to see her get hurt, but may have to watch her carry on with her partner when you think she should leave him or have him arrested. As her friend, make sure you offer her something the abuser doesn’t. For example, if he tells her what to do all the time / tells her that her decisions are wrong, it’s no use you doing the same.
Some helpful things to say:
• Domestic violence is totally unacceptable. Every woman has the right to live her life free of violence, abuse, intimidation and fear.
• Domestic violence is very common. One woman in four experiences domestic violence at some point in her life.
• Domestic violence is very dangerous. Each week in the UK, two women are killed by a partner or ex-partner.
• Domestic violence is about power and control. Abusive, violent and sexually abusive behaviour is wide-ranging and subtle in what it tries to achieve (lots of abused women find this helpful in identifying the range of tactics)
• Domestic violence is intentional and instrumental behaviour. It is about scaring a woman into doing something that she doesn’t want to do, or scaring her out of doing something that she does want to do (This is often difficult to accept but can be gently challenged by pointing out that he can control himself with his boss or when the police turn up or pointing out that it is mysteriously only ever her possessions that get broken etc)
• The abuser is 100 per cent responsible for his abuse. His abuse is his problem and his responsibility.
• It is not your fault. No woman deserves to be abused, regardless of what she says or does.
• A man can change if he wants to. His behaviour is within his control and he can choose to stop. He could call (anonymously) the national perpetrator's phone line
• You cannot change him. He himself is the only person who can stop his violence.
• You don’t have to put up with it. A woman has the right to safety and respect, to put herself and her children first and to focus on her needs.
• You can increase your safety. If the man is intent on being violent, she will not be able to stop him. However, there are things she can do to increase her safety. Contact the National Domestic Violence Help-line (0808 2000 247)
Help build your friend’s self-esteem. Remind her of her good points, challenge her if she puts herself down or blames herself, praise her for every step she takes, and let her know she has your support.
On a practical level you could:
• agree a code word or action that your friend can use to signal that she is in danger and cannot access help herself
• offer to keep copies of important documents and other items (such as an overnight bag) for her. That way, if she has to leave in a hurry, she doesn’t have to waste time collecting important belongings.
• together or on your own, find out information about local services and help.
• offer any practical help you are able (and feel comfortable) to give, such as the use of your telephone or address for information or messages, keeping spare sets of keys/overnight bags/important documents for emergencies.
Planning ahead doesn't commit her to a decision but it will help hugely in an emergency and will at least start her thinking of leaving as a possibility.
Some good questions to ask:
• What can I do to help?
• How has his behaviour made you feel?
• How is it affecting you?
• How have you been coping with the abuse?
• What can you do to make yourself safer?
• What are you afraid of if you leave?
• What are you afraid of if you stay?
• Do you know when an incident is going to happen? Is there a pattern?
• What’s your worst-case scenario for yourself/your children?
• What are your worst fears for yourself/your children?
• What do you already do to protect yourself/your children? (This is a good chance to help her see how much she may already be doing to stay safe.)
• Which of the things you do to protect yourself/your children work in practice, and which don’t?
• What personal strengths do you have that help you to deal with this situation? (Explore how can these be increased, for example, by raising her self-esteem, self-belief and ability to trust her gut instincts. Assume she is resisting and is doing things to try and stay safe - she is very unlikely to just be passively accepting it. Just because the things she is doing aren't working doesn't mean she isn't doing them)
• What external resources are there to help you cope (support networks of friends and family, access to money, access to alternative accommodation and so on)? How can these be increased?
• Can I help you find out about what other choices might be available?
• Which options would be most realistic for you? What do you see yourself as actually being able to do? (Focus on those.)
Finally, although most abuser's are charming to everyone but the person they are abusing, their dangerousness tends to increase as they sense their control slipping so whatever else you do, be sure to keep yourself safe. If at any point you fee overwhelmed, do be aware that DV services will be happy to support you in supporting your friend.
Good luck - and 
for being such a caring friend.