If someone is a witness to another person ( that they are familiar with) being beaten, is it against the law if they don't try to stop it or call the police?
No, not generally. Arguably some kind of duty may exist if there is a special relationship, such as parent-child, for purposes of potential civil liability if not criminal. Another, practical problem could be the risk a witness bears of being accused of a crime as a result of being there, such as conspiracy. I have represented people accused of conspiracy to commit [assault] based mainly upon there presence when an assault occurred.