Certainly, certainly.
As for the “prohibition” involved, I told him from a practical standpoint that it’s not more severe than missing tefillin for one day, but there is a great deal of impurity in it, and afterward it becomes hard to grow in Torah.
I just wanted your opinion about how seriously this should be regarded, and perhaps what exactly makes it so serious.
*P.S. In principle I referred him to you so he could ask you directly; he called me, we had a long conversation, and he feels very bad about himself..

Did she immerse, so that she was not a niddah? Usually unmarried women do not immerse. In that case there is the severe prohibition of niddah. Without that, there may be neglect of the positive commandment of kiddushin, and apparently “there shall not be a harlot” does not apply here (because she is not designated for this). There is also room to discuss the prohibition of wasting seed, since this is intercourse not for the sake of having children (which they permitted with one’s wife, but not with another woman). But simply speaking, that is a rabbinic prohibition.
If he feels bad about himself, that is very good. It is leverage for repentance. Of course, not a bad feeling that destroys him from within and makes him despair.

Hello Rabbi,
On the face of it, from the words of the Vilna Gaon it appears (siman 26, se’if katan 8) that with a concubine there is neglect of the positive commandment of kiddushin, but with an unmarried woman with whom he has an ongoing relationship, though not in exclusive seclusion like married life, perhaps there is also no neglect of the positive commandment?
And according to the Chelkat Mechokek, who wanted to infer from the rabbinic prohibition of seclusion with an unmarried woman to a prohibition of intercourse—how would he explain Nachmanides’ permission regarding a concubine, where he mentioned no prohibition at all, not even a rabbinic one?