Assuming the questioner does not believe in Jewish law at all, is not a theist, and has no doubt about it whatsoever: morally, in your view, should he inform them? From his perspective he is not harming them at all.

Definitely yes. First, one must respect their right to act according to their own understanding. And besides, there is always the possibility that he is mistaken and they are right.

As for the first part of your answer, I do in fact have a moral intuition like that, even though I don’t have a reduction of it to harm to another person, at least in my view. This is one of those cases where I’m not sure whether the intuition is correct — whether there is moral force to a non-harmful act — or whether it’s just an illusion arising from similar cases in which someone is actually harmed. It’s hard to identify a criterion that distinguishes between these cases.

Let me preface by saying that my two claims work together. But even regarding the first one, you are in fact harming him. A person wants to act autonomously according to his own understanding. It does not matter whether the eternity within hod remained intact (or does not exist at all). You yourself would not want others to do the same thing to you, either as a consequentialist argument or as a categorical imperative.

I don’t understand what you mean when you write, “you are also harming him.” If he is not aware that he is not acting according to his own understanding, then it does not harm him. A categorical imperative and consequentialist morality do not change that fact. Having said that, I do feel that it is immoral even though I am not harming him — his experience is indifferent to my action — and I don’t know how to tell whether this is valid morality or not.

His experience* is indifferent.

If you slander someone all over the world but he never finds out about it, is there no moral problem there? He is not harmed in any measurable way; it’s just that everyone in the world thinks he is scum, and that’s it, while they continue to treat him as usual. In my view there is an obvious moral problem there. Something like respect for the dead, for example. Even if there is no survival of the soul, that person during his lifetime would have wanted people not to abuse his corpse, or him himself, or his children, after his death.

If people would relate to him in exactly the same way even if I had not slandered him, then literally he was not “harmed.” I don’t see how the concept of being harmed can be understood otherwise.

If the questioner finds no meaning in moral life, why is he interested in being fair?
And what is the connection between “the God of Maimonides” and the meaning of Torah and morality?

For example, if a very wealthy person has money stolen from his bank account, even if he doesn’t notice and it does not affect him in any way de facto, it is still called harming him. There are aspects of a person through which he can be harmed without that translating into de facto suffering or distress.