My thought wasn't that he wouldn't have anything true to say. It was that if he's still defending good and evil as obviously existing, in that context, he's far enough behind me on the issue that I can safely assume that he doesn't have anything major to teach me, and that what he says is untrustworthy enough (because there's an obvious flaw in his thought process) that I'd have to spend an inordinate amount of time checking his logic before using even the parts that appear good - time that would be better spent elsewhere.

Many people here appear to have a similar epistemic immune response to people who bring up God in discussions of ethics. I'm surprised it's considered an issue in this case.