Ethically, should you report cheating that you learned of indirectly?
For many types of moral situations, university rules actually require that you inform a superior as soon as you find out yourself, or you are also in violation.  It may be that the classmate who informed you didn't have the civic courage to report it but knew that you did.  It is precisely to break cycles of no one saying anything that the rules are set up in the way I just described.  

Or of course it may be that they were lying.  As long as you passed the full information to the professor about how you knew what you knew as well as what you believed you knew, it is now the professor's responsibility and not yours.  If there's anything you haven't already told the professor, I'd suggest contacting them again.