It’s not your job to do this. People are responsible for their own decisions, and that includes who they choose to have a relationship with.
Consider this: would you want to be with someone who would choose “the wrong guy”? Because if they chose you that means you’re the wrong guy.
You can’t swoop in and save someone from themselves, no matter how much you care about them. If they decide to get into a relationship with a horrible person that is their choice and in my experience there’s nothing you can do until and unless they decide to leave.