I would think it would be illegal to call a student, who doesn't know any better, and threaten to ruin their life, especially if the threats you're making aren't actually true.
In any case, the obvious next step in the student's situation is to contact a lawyer, and see if there's any merit to the threat. For all he knows, a lawyer would have told him that it's a totally empty threat and he shouldn't worry about it. It is naive to listen to the word of someone who is obviously opposing you.