A son going off the derech is the mother's fault

So states the Kav HaYashar on parashat Pinchas. He tells over the following presumably apocryphal story. (We last saw the Kav HaYashar as endorsing the segulah ring.)

{Update: Just to be absolutely clear, I do not agree with what the Kav HaYashar is saying. And for a great rebuttal / clarification on the Jewish approach to this, see Rabbi Yakov Horowitz's article here.}

First, he cites the pasuk in Mishlei {13:24} that
"כד  חוֹשֵׂךְ שִׁבְטוֹ, שׂוֹנֵא בְנוֹ;    וְאֹהֲבוֹ, שִׁחֲרוֹ מוּסָר.
24 He that spareth his rod hateth his son; but he that loveth him chasteneth him betimes."

and that any man who chastises his sons loves him. Then, he continues as follows:

"The son comes to an evil behavior, chas veshalom, because of his mother: 

This meanwhile is not so for a woman, for she is softhearded and does not wish the father to hit his son, or the melamed. And while the child is yet young his mother gives him all he wants and all his heart desires. And afterwards, when the child grows up, he wants this from others, and afterwards comes to an evil end, and all is caused by the woman. 

And I heard a proof to this from a wondrous incident. That the Ramban wrote in one of his sefarim of derush and said, 'any son of an Israelite who turns to apostasy and worships idolatry of one of the nations, then you need to know that that son is not from the seed of Israel; for certainly he is from the seed of the nations or the son of a swap {?}.' 

And it was not many days that one of the sons of the Ramban became an apostate, at which the Pope sent after the Ramban and said, 'see your derush which was recently printed, and now your children have stumbled in your words.' 

And then, the rav, the Ramban went to his house, despondent and wrothful. He say and cried on the ground, and afflicted himself with great afflictions, and was pained more regarding the derush which he had printed than the pain of his son who had turned apostate. And he did not eat or drink for some days, until his wife came to him and spoke to his heart, that he should not be in pain. For one time, she went to the house of immersion {=mikveh} which was far from her house. When it was dark, she she wished to go to her husband's house. Then, the sar {=government official} saw her great beauty, and the sar commanded his servants to take her to his house. And the sar raped her, and from him she became pregnant. And if you don't believe {?} me, here is his finger, still in my possession, which I bit off his finger with my teeth. 

And when the Ramban heard his wife's words, he arose from the earth and greatly rejoiced, and said to his wife, 'You have comforted me'. Then, immediately he girded his loins like a man and went to the Pope and told him these matters. And the Pope immediately sent for this sar, who come to him, and he had on his hand a glove (gauntlet) which they call a handsheik, and the Pope commanded him to remove the gauntlet from upon his hand, and the sar did not wish to do so. Then the Pope commanded his servants to remove the gauntlet, which they called hansheik, from his hand by force. Then the Pope saw that he was missing a finger from his hand, and asked him the meaning of this missing finger. And then the sar admitted that once he raped a married woman from the Israelites, and the woman bit off his finger with her teeth. Then, the Ramban said 'See that our Torah is true, the words of the Sages are true, and the words of Hashem are true.' "

I am not sure of the connection to parshat Pinchas. Perhaps the genealogy of Pinchas to Aharon; perhaps Hashem's testimony about the shivtei Kah that they were not raped by the Egyptians (see Rashi on 26:5).

This is most likely not a true story. As noted at Daas Torah:

The dialogue seems improbable, not least because the papal court was in Rome at the time (unlike shortly after the Ramban's death when it moved to southern France).

Perhaps it is only to the modern Western reader, but the Ramban does not come off as a great role-model in this tale. He is aggrieved more about his derush than about his son becoming a meshumad? On hearing that his wife was raped, his reaction is not great sadness that this happened to his beloved, and concern for her, but great joy?!  I don't believe such motzi shem ra about the Ramban.

What likely happened is someone made up this story about the Ramban, to convey his own religious worldview. He ascribed this to Ramban, maybe because of his famed disputation or maybe because he found this idea of not being of Jewish seed in a derush from the Ramban. (I don't know if the Ramban indeed says this anywhere. I do recall an idea floating around that if a child goes off the derech and converts to Christianity, it shows that his ancestors did not stand at Har Sinai. What this says about various great Rabbonim whose children or grandchildren converted is difficult. It could be that this was the prompt.)

Zera Yehudim in this story I suppose means from Jewish sperm. Because the child was nonetheless Jewish, because his mother, who was raped, was Jewish.

Other problems with the story and the message from this work of mussar is that it will encourage abusers. A father is taught here that he is right to hit his child. (And for those who overdo it, beat his child.) And that it is wrong to refrain from doing so. And if the mother protests and tries to prevent him from doing it, she is going to cause the child to end up in a תרבות רעה, chas veshalom. Women, in being nice to their kids, are spoiling them. And if the kid does eventually go off the derech, it won't be because of the abusive father but because of the gentler mother. Either because she coddled him too much, or worse, based on this story, because she was unfaithful to her husband. For not every case of pregnancy from a non-Jew is the result of rape, and if she denies sleeping with a non-Jew, the husband will still suspect her of deliberate unfaithfulness. Excellent mussar, here!