If we imagine the circumstances under which he got arrested, either his girlfriend (or another family member) called 911, or his neighbours did because there was a loud disturbance. In either case, he would have only been arrested for assault if their was physical evidence of assault when the police arrived, i.e. a black eye, a bruised neck, bleeding, scratches, etc. If this is true, they'll have all the evidence they need to pursue the charge, and she would only risk perjuring herself if she gave false evidence to try to protect him... which she probably wouldn't do anyway if she was the one who called 911. You don't just call 911 the first time you have a yelling match with someone, there was probably an escalating pattern of threatening behaviour/violence.