What? No! Why would you even think of beating her? Even if you do beat her, your brother's still been emasculated. After you tell your brother, what you can do, if you must help, is find a way to restore your brother's sense of masculinity and identity. As long as he's confident and secure, she can't hurt him. This can involve helping him reconcile with his wife, or getting him some younger, more attractive women (i.e. divorce). Ideally, don't beat anyone.