The lesson consisted of a wrong note. Instead of a C# at bar 2 he played a C-natural.
Based on pure ear logic, which is to say, logic not compromised by any theoretical concepts, the C-natural is more logical than the C#. The proof?
For one thing, it is exactly what Beethoven himself does at the second ending. Secondly, it exposes the malaise of this sonata's opening, which is always awkward. Not that I would have said as much; I simply avoid the piece as I suspect many pianists do.
It is not the first time I have fallen off my chair at the insights into Beethoven furnished by this young man's ear-pure listening.
With whom did I study Beethoven? With my student.