Chevis_
That is an odd one. I’m not a lawyer, but my guess is that a lawyer would say there is no case there. I’m not sure if you can be sued for telling the truth. Again, not a lawyer, and it all depends how it is construed.
Now onto the more grey area as to whether your colleague should have informed clients he was leaving. If he is close with said clients then yes, but if not, then I would say he probably should not have, especially not from his former employer’s email. That is up to the company he is leaving to decide how they want to handle it. If they want to lie, it will eventually come out and come back to haunt them. Your friend had every right to announce he left to friends, on linked in, face book and what have you and if former clients see it, they see it. If he sent the emails from his own account and his own computer, that lessens it, but you don’t want to do the equivalent of turning over the tables on your way out the door when you leave somewhere.
Obviously the former employer should not have lied, and sounds like he is taking letting out his frustration on being caught out.