Don't disqualify yourself from being a good listener because you have not dealt with this type of situation before.
Remember that the most important role you can play is that of a caring person who is there to listen to and support your friend . . . without feeling like you should take the problem on as your own.
Don't make excuses for someone else's inappropriate behavior.
Use statements that bring order to the situation and serve as a reality check, for example:
"You know, I've had a lot of boy/girlfriends and none of them ever made marks on my arms like that."
"Regardless of the circumstances, no one should ever hit you or throw things at you."
"Just because someone has been drinking doesn't mean they aren't responsible for their actions 100% of the time."
"No one has ever told me to dress or act a certain way."
"You shouldn't have to stop hanging out with your friends because he or she expects you to account for every minute of your time."