Hidden Opinions

You deftly met your clashing ethical obligations to your student and the Grouchy Professor (my least favorite Disney movie, by the way). By speaking to the former, you protected her from avoidable professional harm. By keeping mum about the letter's contents, you respected the latter's confidentiality.

Your student simply assumed the letter to be derogatory. Even a well-meant reference letter can be ineffectual, a good reason to eschew it in the future. It could be too brief to be helpful or lacking in specifics or clumsily written or covered with soup stains. The Grouchy Professor's own reputation could make his endorsement counterproductive. Having made that assumption, the student imprudently confronted the professor, stirring up this hornet's nest and placing you in jeopardy. She should have known better.

UPDATE: The student's approaching her reference provider induced other people to make more substantive complaints about the professor. He subsequently decided to retire.