This is the original ending to the poem:
"Said the frog: "I tried to teach her,
But she was a stupid creature -
Far too nervous, far too tense.
Far too prone to influence.
Well, poor bird - she should have known
That your song must be your own.
That's why I sing with panache:
"Koo-oh-ah! ko-ash! ko-ash! "
And the foghorn of the frog
Blared unrivalled through the bog."
I cannot understand why you wish for an ending in which the "nightingale is justified as a gullible creature", since the ending above more than supports that idea. The frog states that "she was a stupid creature ..." and "Far too prone to influence".
These statements prove that the nightingale was gullible. She trusted the frog too easily and took him at his word, believing everything he said and even feeling flattered that he, such an 'iconic' figure, should take an interest in her and agree to help her improve her skill.
The frog suggests that she should have had more confidence in her abilities and should not have allowed herself to be so easily misled: '... she should have known that your song must be your own."
Maybe you could replace the word "stupid" in the original version with "gullible" - which would obviously make it clear that she was naive.
As a suggestion, the poem could end as follows, which would, I hope, clarify the matter:
"Said the frog, "In my attempts to teach her,
I found her a most gullible creature.
She believed everything I said,
and because of that, she now is dead.
The sad truth is, she should have known
that everything you believe must be your own."