I think this is very important too

Friends & Family
Yes, it is your business
Maybe he’s(she's) your friend, your brother-in-law (SIL), your cousin, co-worker, gym partner or fishing buddy. You’ve noticed that he (she) interrupts her (him), criticizes her (his) family, yells at her (him) or scares her (him). You hope that when they’re alone, it isn’t worse.
The way he (she) treats her (him) makes you uncomfortable, but you don’t want to make him (her) mad or lose his (her) friendship. You surely don’t want to see him (her) wreck his (her) marriage or have to call the police. What can you do?
Say something. If you don’t, your silence is the same as saying abuse is ok. He (she) could hurt someone, or end up in jail. Because you care, you need to do something… before it is too late.
What Can You Say or Do?
Taken from http://www.ndvh.org/educate/abuser_quiz.html