Imho this particular case (and lots of other cases) the root cause is absolutely taaveh.
So how is any intellectual argument going to help with that?
The issue is since it is taaveh they have a guilty conscience and try covering it up with all sorts of theological questions. Once they start reading apikorses and the likes, since the rock solid foundation of yiddishkeit is missing they get all excited and start believing there might be some truth to the garbage. And so it continues...if R"L there is no schar v'oinesh then let's do as we please...
It might be missing, and it might not be missing. Taavos are very strong, sometimes they can overpower even someone with strong foundations. And once things begin, one never knows where they can end. Maybe the right approach would be to start talking to him about his kids, which I would assume he loves, and how he would like them to grow up.
So if I can provide this fellow with some material on the basics #1 he will see none of the questions were born today and however strong the questions Judaism is stronger and #2 even if I won't get him to that point of realizing the truth it'll definitely give him some guilt feelings which might stop him from slipping more
You are making some good arguments here, which leads me back to the conclusion, that guidance from an expert, rather than from friends on DDF, is what's called for.