I'm guessing not in the UK--which has markedly stricter (and often criticized) libel/defamation laws than the U.S. where you presumably "practice."

Also, the facts as he presented them do present a prima facie case for libel/defamation in the state I practice in--which I think mirrors most other states.  He'd have a damages issue, and the litigation would be cost prohibitive for a student, but he would not be "laughed out of court."

OP, you should sit down with a barrister and discuss the situation.  Worth paying for a couple hours time to get the real lay of the land.