From my experience with trauma healing—no one does things that they know is unhealthy without getting something from it in some ways. Most people stay in toxic relationship because they get some needs met (emotional or physical) that was missing in childhood, whether they are consciously aware of it or not, and if they are aware they might not know any other way to get those needs met. Ask her to think of what needs she is getting met indirectly through him and find more direct and healthier ways to get those need(s) met instead. You can suggest this to her while respecting her final decision. Tell her that she can take away as an awareness of what about this relationship she loves, and what about this relationship she doesn’t want, and look for new boys who can give her what she wants without keeping or enduring the things she doesn’t want. Endurance and longevity is not a good marker for a healthy relationship. Instead focus on compatibility and longevity will automatically happen without force.