Does the emission of semen without sexual intent during an obligatory fast, such as for expiation, necessitate invalidating the fast and redoing it?

A man and a woman who are not mahram (definition) to each other are not permitted to address one another unless there is a necessity and the absence of fitna (temptation), and without softness in speech. The emission of semen with desire invalidates the fast. As for the interruption of the continuous expiation due to something that happens to a person unintentionally, there is a difference of opinion:

Abu Hanifa: The continuity is interrupted even by illness or menstruation. The majority: What interrupts the continuity is what a person does by choice, not what happens unintentionally.

The preponderant opinion is that of the majority, because a person is not burdened with that which they have no power to avert. Therefore, if your friend did not intentionally seek pleasure in the conversation and it was not his habit for ejaculation to occur merely from talking, then his fast is not interrupted, and he should build upon what he has already fasted, keeping in mind that it is most likely what was emitted was madhy (pre-seminal fluid), which does not invalidate the fast in the first place.