It is actually a short phone conversation in which a person calls his friend to congratulate his 50-year-old birthday and also pays compliment to his achievement. Probably Ну что ты is some kind of denial here, I think.
My Chinese translation of "Нет, в самом деле" says something like "No, it's true." And I just found a source here,
в самом деле, maybe it's meaning 2)?