Maybe his new boss called for a reference. I know in some states its illegal to say anything beyond
Yes he was employed here
and whether or not he is eligible for rehire.

Though I have had people call me and try to press me for more info. Its possible the new boss knows the old boss and they got chatty.

Unless you can prove it was all lies, and get someone to stand with you and back you up you are pretty much out of luck.