He uttered the word of divorce (talaaq) whilst explaining the rulings on divorce and its wording to his wife

Firstly: 

If you were speaking by way of explanation and giving
examples, and you had no intention of divorce, then no divorce has occurred,
because the Prophet (blessings and peace of Allah be upon him) said: “Deeds
are but by intentions and each man will have but that which he intended.”
Narrated by al-Bukhaari (1) and Muslim (1907) from ‘Umar ibn al-Khattaab
(may Allah be pleased with him). 

Hence the wife of a teacher or faqeeh who says, when
explaining and discussing issues of divorce, “My wife is divorced”, is not
actually divorced. The same applies in the case of one who utters the word
of divorce without meaning to (a slip of the tongue). 

Zakariya al-Ansaari (may Allah have mercy on him) said: The
third condition (of divorce being valid) is the intention of divorce. So it
is stipulated that he should intend to utter the words and intend the
meaning of the words, both together, so as to break the bond of marriage…
Because what matters is intention to say the word and intention of the
meaning thereof, both together. The reason why there is a second condition
for divorce to take place, namely intending to divorce one’s wife, is so as
to exclude other scenarios such as when narrating that someone else divorced
his wife, or when a faqeeh is explaining how divorce takes place, or calling
by name a woman who is called Taaliq (which means divorced). 

End quote from Asna’l-Mataalib, 3/280 

What is meant by calling by name a woman who is called Taaliq
is saying, Yaa Taaliq (O Taaliq) 

In Sharh Manhaj at-Tullaab ma‘a Futoohaat al-Wahhaab
(4/336) it says: 

The condition is that he should intend divorce and intend to
utter the word talaaq for what it means, in the sense that he intended to
use the word for that purpose. For example, divorce does not take place on
the part of one who asks some people for something but they do not give it
to him, so he says: “I divorce you all,” and his wife happens to be among
them but he did not realise that. Nor does divorce take place on the part of
one who is telling how someone else divorced his wife, such as if he says
“So and so said, ‘My wife is divorced.’” End quote. 

And Allah knows best.