What is incumbent upon one who swore that a specific person is a reciter of the Quran and was certain of it, then it became clear to be contrary to his statement?

Your friend's oath is considered a "laghw al-yamin" (a vain or unintentional oath), because he thought that the reader was the one he swore about, but it became clear that it was not him. So, there is no expiation for him and no sin. And his statement "May his hand be cut off" is not an oath, and he must repent to Allah for this supplication.