You tell the girl and her family secretly. But also tell them that not to mention who informed them about the guy’s character. Tell them to do istikhara. I know when one already knows what another person is like, then u may say there’s no need to do istikhara. But Allah SWT knows everything, while our little minds don’t know much. Maybe he can become a very good person after he gets married? But if not, then that istikhara would show them the result. Ur in-laws know about the guy’s character?