Probably not much, since there argument for a moral standard will probably be that God is the objective basis for the good or the moral and they will say that without God there is no objective basis. So they will most likely reject anything you propose because it will be relative and subjective. And even if you do present very good arguments and well laid out logic fundamentalists will still reject you. I personally think there is a very good case for morals that can be made. But there is an old saying “that you can’t reason someone out of something that they weren’t reasonef into in the first place.” QED ?

I tell them they are wrong in every possible way, and if it starts a discussion I explain the scientific foundation of morality and it's evolutionary necessity in a social species, often haranguing them until the stop talking and go away.

occasionally, when I'm just not up to it, I say "Okay" and walk away.