What not to do

Don't blame the victim
And don't ask judgmental questions such as ‘What did you do to make them treat you like that?’ or ‘Why don’t you just break up with them?’
Don't focus on trying to work out the abuser’s reasons for the abuse.
Concentrate on supporting the victim and discussing what they can do to protect themself.
Don't be impatient or critical of the victim if they are confused about what to do or if they say that they still love the abuser.
It’s difficult for anyone to break up a relationship, and especially hard if they are being abused.
Don’t try to maintain a friendship with both the victim and the abuser.
If you try to support both parties, you’re not going to be much help to either. They need to be able to talk to someone who believes them, who will not pressure them to ‘see it from the other person’s point of view’, and who would never encourage them to get back together with the abuser.

Most importantly, don't give up on your friend. You might be their only lifeline.