The question is theoretical.
If you want it to be realistic, then I’m asking: what is your policy?

My policy is to use valid witnesses, for several reasons:
1. You are supposed to enable a person to marry according to the law of Moses and Israel. If he decides to be a sinner, that is his problem (“feed the wicked and let him die”). Something similar applies to saving Sabbath desecrators on the Sabbath through Torah-level prohibited labor (they will not keep many Sabbaths, but nearly all halakhic decisors ruled that they should be saved. You enable them to keep Sabbaths, and what they actually decide to do is their problem).
2. Transparency. You are not supposed to make decisions for a person in his place. You can of course refuse to marry him off, but not tell him that you are marrying him and then not actually do it.
3. As far as mamzerut goes, it is a problem in any case (though of course this is a very rare case), but as far as transgressions go, if he is not obligated in Torah and commandments, then his transgressions are not transgressions.

What do you mean by saying that if he is not obligated in Torah and commandments, then his transgressions are not transgressions?

Whether he is like a child captured among non-Jews or under compulsion (his mind compelled him).

Why is mamzerut a problem in any case?
If there is no betrothal, then there are no mamzerim. Right?

And regarding transparency, you can tell him that you are not marrying him in a halakhically valid marriage, so that if the day comes and his wife cheats on him, then at least her children will not be mamzerim.

Yehuda, see here:

בעניין הכשלת חילוני בעבירה

Reuven,
But if there is betrothal, there are mamzerim. I explained my view that valid witnesses should be used.
Your suggestion regarding transparency reminds me of the midrash on “I am Esau your firstborn.” Begging your pardon, those word games do not help in any real sense. When you lie, you lie. And in general, from that standpoint it would be preferable not to marry anyone in a valid marriage, and thus mamzerim would be prevented entirely. After all, even among secular people, mamzerut is an extremely rare matter (because people use contraception to avoid giving birth from someone who is not their spouse).

One can hear what you say in that article regarding a child captured among non-Jews. But what you wrote there at the beginning—that even without that category, the very fact that he does not believe at all means that his act does not in practice bear the status of a transgression—is very strange, and of course one cannot derive from this regarding an act of refraining from a prohibition.

The language of Maimonides in the Epistle to Yemen is: “This Torah—no one of the seed of Jacob will ever be able to escape or be saved from it, neither he nor his offspring nor his descendants, whether willingly or unwillingly. But he is punished for every single commandment that he neglected from among the commands, that is, from a positive commandment, and he will also be punished for everything he transgresses from the prohibitions. And it should not enter one’s mind that when he commits grave sins he will not be punished for the lighter ones, so that he should make them ownerless for himself. But Jeroboam son of Nebat, may his bones be ground to dust, will be punished for the calves with which he sinned and caused Israel to sin, and likewise he will be punished for having neglected the commandment of sukkah on Sukkot. And this principle is one of the foundations of the Torah and the religion; they taught it and inferred from it.”

I agree with every word of Maimonides. What does that have to do with what I said? I agree that every Jew is obligated in all the commandments and cannot escape that, and I agree that he will be punished if he transgresses them (only if done intentionally, of course).

Because I understood from your words that the transgression of an apostate who does not believe at all does not bear the status of a transgression. But if Maimonides is speaking about an apostate and says that he will be punished for every detail, then it is considered a transgression.

I explained the difference between the apostate of former times and that of our day.

Implicit in the Rabbi’s words is as though a secular person who asks a rabbi to marry him intends to be married according to the law of Moses and Israel.

As I understand it, all he wants is a ceremony with a rabbi, and he is really not interested in whether this is done according to the strict rules of Jewish law.

Do Maimonides’ words in the Epistle not contradict what he says in the Guide for the Perplexed and the Mishneh Torah?
After all, according to his words, the purpose of the commandments is to direct and prepare the soul (or the intellect) for attaining the intelligibles.
A person who sinned severely distances himself from the intelligibles, does not attain them, and does not merit the World to Come.
In that situation the lesser transgressions have no significance, for in practice he is not punished for them.
A person who attained the intelligibles despite not observing the commandments (Aristotle, for example) certainly is not punished at all.

About contradictions such as these, I do not know how to answer. I do not know whence Maimonides derives the purposes of the commandments. But he certainly holds that all of them must be observed, and some of them are apparently a basis and preparation for engagement with the intelligibles. Therefore, without them, even engagement with the intelligibles is incomplete.