Do...
... talk to the abuser if you feel safe doing so
Talk about your concerns and refuse to accept any excuses. Be clear that you are still a friend, but you disapprove of the behavior.
Be there, listen, and stay there
You may feel like a broken record but you'll be surprised at how much of what you are saying is getting through.
Do recognize and praise the good behaviors
Do encourage them to be honest
Show your support when they are.
Do help them clarify their feelings
Explain that possessiveness and jealousy are not love.
Do understand that abuse is a CHOICE
Help your friend understand this. Abuse is a learned behavior.
Do encourage them to talk to a counselor
Go with them if that's what it will take.
Don’t...
... is never okay. Not because of a bad day, because someone was drunk, and not because someone was really angry.
Don't get stuck in the middle
Don't be a mediator or offer to be a "go between" for the couple. You are there to help him/her bring about change in an abusive behavior.
Don't cut off your friend
Reject the behavior, not the person. Be very clear about this. Explain that their abusive actions are bad, not that they are a bad person. Choose your words carefully.
Don't encourage abuse
Laughing at degrading jokes or put-downs signals acceptance of the behavior.