The commentators discuss your question.

The Yosef Da'as explains, based on the Rambam (in Hilchos De'os 7:5) that the nature of his wife was already public knowledge, and thus mentioning it did not involve Lashon ha'Ra (see Rambam there).

The Chafetz Chaim (Hilchos Lashon ha'Ra, K'lal 1, Be'er Mayim Chaim 13) explains that the son was asking what defines a "bad wife," as mentioned in the verse. Rav Yehudah explained to his son, who certainly recognized the nature of his mother, that the verse could be defined by the nature of his mother. When one has positive intention (and does not intend to mock or insult anyone) and says the truth -- such as to explain the verse -- it is not Lashon ha'Ra.

Finally, the Margoliyos ha'Yam quotes the Eshel Avraham who explains that Rav Yehudah was educating his son about the responsibility of a husband to direct and guide his wife, patiently and caringly, in her ethical and spiritual development. He was saying that his wife, by nature, had bad traits, but that she had acquired the ability to overcome those traits.

Y. Shaw

D.A.F.

Jerusalem