She should have reiterated: boy could have been killed. This was the alternative to completely taking away his brother's, family's, and (genuinely beloved) sister in law's future due to his own childishness; there would have been no recovery for any of them from this. His mercy was to let others suffer for his mistake, and realistically, again, he's lucky he wasnt killed on the spot to begin with.
Realistically this was the best case scenario, especially because him falling for that ploy so easily shows that he's not well suited for the palace. Better be flogged and kicked out now than have himself and everyone related to him executed later