Some girls like that will give you the most love and make you the happiest man alive if you are able to make them happy and make them realize they can do better than what they have been put through . But I guess to each man their own .
It's not a matter of opinion, you have to look at things objectively.
You do realize this is exactly how the girls who go to jerks think. They think that if they show them enough love that the jerks will change and become more lovable.
I'm not saying you're not allowed to tell a girl what you think, by all means do tell her that you think she's dating a jerk and that she can do better then that. But if she doesn't follow up by taking action on her own (breaking up with the jerk and seeking a good guy) then it's not going to change. And anything done afterwards will simply be a waste of your time; time that could be used to date other girls.