Question

what was the reaction of the Frog at the death of the nightingale

Answer 1

The frog insulted the nightingale. He said she lacked confidence and she was easily influenced by anyone. And he also said that her song should be her own, that is ,her way of singing should be her own

Answer 2

In the end when nightingale died, the frog was very pleased that he now able to rule bingle bog once again and own her song.
The frog reaction was very brutal reminder of how someone can take advantage of others and how being too trusty can be disastrous. Here his reaction shiw his cruel and crafty nature.
Hope this will be helpful :)