Don't ignore your friend or say you'll only be a friend if s/he gets out of the abusive relationship.

Don't put down the abuser.

Don't tell your friend that her/his boy/girlfriend is a terrible person. Instead, talk about how you think relationships in general should or shouldn't be. Remember, your friend cares about their boy/girlfriend - bad mouthing the partner will most likely lead your friend to stop talking to you about the relationship.

Don't talk to your friend about the abuse in front of her/his boyfriend or girlfriend.

It may not be safe for your friend to talk about the abuse in front of her/his partner. Don't talk to the boy/girlfriend about the abuse AT ALL without your friend's permission.

Don't put your friend down for being in an abusive relationship.

Don't say things like, "I can't believe you're stupid enough to stay with someone like this!"

Don't tell your friend what to do.

Don't assume s/he wants to leave or that you know what's best for your friend. This may make your friend afraid to disappoint or anger you. The abuser is already trying to control her/him. You don't want to try to control your friend as well. This just reinforces your friend's role as a victim.

Don't blame your friend.

Don't ask your friend what s/he did to provoke the abuser. This will only reinforce any feelings of self-blame that your friend may already have and make it harder to expect the abuser to take responsibility for the violence.

Don't tell your friend s/he has made a bad decision.

Your friend is the only one who can decided what is best for her/him and what s/he is ready and able to do. Your friend probably has a lot of doubts about her/his ability to make good choices. You don't want to make it worse. You also don't want your friend to stop talking to you because s/he is afraid you won't approve of what s/he has decided.

Don't try to physically protect or rescue your friend.

Abusive relationships can be very dangerous. You can't help your friend if you put yourself in danger or get so involved that you lose your perspective too.