I would reply that you in turn are assuming that whoever wrote John's Gospel was not there observing the conversation. Assuming that he was not based on nothing more than the absence of an explicit claim to the contrary amounts to an argument from silence.
It was at night, but the assumption that Jesus and Nic were the only two present may not be correct:
...The Method Of Teaching in these schools may be easily collected from the Gospels and Acts. The Doctors or Teachers generally sat. Thus our Lord sat down previously to delivering his sermon on the mount (Matt. v. 1.); as Gamaliel also did in his school. (Acts xxii. 3.)
Sometimes, however, the Jewish teachers, like the Greek philosophers, were accustomed to have their disciples around them, wherever they went, and to discourse, as occasion arose, on things either human or divine. In this way our Lord delivered some of his most interesting instructions to his apostles. Allusions to this practice occur in Matt. iv. 20. x. 38. xvi. 24. Mark i. 18. xvi. 24.
An introduction to the critical study and knowledge of the Holy Scriptures. , Volume 2
Thomas Hartwell Horne
Cordially,
So is assuming that he was.