I would say youth and stupidity.
I had quite a sheltered upbringing, so didn't have my first boyfriend until 22. We had a conversation one day about men hitting women and was surprised when he said, he couldn't promise he would ever hit me. Had a man said that to me now, I'd be off like a shot.
Anyway, he did end up hitting me and once it was quite bad that he gave me a black eye. I had to go to hospital and he was arrested. I didn't see him for a few weeks, but like a fool went back to him, his old behaviour soon returned and three months later, I finally saw sense and left him for good.
Hopefully she will eventually come to realise she will never find happiness with a violent man and gets out before it's too late.