Well I don't think you should tell your friend to dump her boyfriend because sahe might think you're trying to control her. You can politely ask her to talk to her boyfriend and maybe she can get him to stop, he might be hurting him unintentionally. If that doesn't work then I would just keep your brother away from him. And if it becomes an issue with every sibling Ben comes across then maybe she will break up with him but I would leave that up to her..
Hope I helped(:
Hope I helped(: