Hitting a person can’t be done according to your choice. You have no right to hit anyone, whatever the reason is. You can talk to your brother. Make him understand that his girlfriend has to understand that she can’t just start hitting your brother. As we are also human beings and sometimes we might miss some important events. You can ask him to talk to his girlfriend about this. Your brother might be in a toxic relationship. So, you have to make this easy for your brother and you can directly talk to his girlfriend. Tell her that what she did was wrong. And this is not acceptable. She has to treat your brother well.