I like Jo's replies. It rings so true.. it explains your friend's flip-flopping actions in this matter.
My best friend was in a horribly abusive relationship not that long ago. For a few years. I was her shoulder to cry on. I even blew up at her now-ex abuser once, screaming and going on a whole (drunken) tirade. And it did nothing. She had to get to terms with the fact that she deserved better, on her own. It got to where I had to distance myself from her.. and not long after that she came back to me saying that she had ditched the loser. We are now both happily married to men who are awesome.. so there is a light at the end of every tunnel.
Oddly enough, my husband was in an abusive relationship, or the tail-end of it, when we met. His ex kept coming back and she was a very violent drug addict who stabbed him with a broken cd in one of her lighter rages.