It is difficult to watch someone you care about be abused. It may be hard to understand why this person stays in that situation.
Understand that once a person in in a victim's role, there may be a multitude of reasons why she/he may stay. Usually fear is the biggest reason. Financial dependency, keeping the family together, feeling hopeless, low self esteem may be a few other reasons for not knowing how to get out.
The best way you can help is to offer your non-critical support:
1. Offer a safe place for this person to go if things get worse
2. Help make a safety plan if this person is willing
3. Offer your phone or computer for this person to access outside help where she/he cannot be monitored.
4. Listen. If you can't offer the previous things, be a shoulder this person can cry on. Sometimes knowing there is someone to turn to, can make a huge differenct than feeling alone and without options.
5. Find out options and resources available for this person. Make them available when this person is ready.
6. Take care of yourself. Helping someone who is in a hard circumstance can be draining on your emotional state also. Make sure you take time to do the things that make you happy.