Statements must be false to be defamatory (http://en.wikipedia.org/wiki/Defamation#Truth http://en.wikipedia.org/wiki/Defamation#Truth). While Yishan's actions

Statements must be false to be defamatory (http://en.wikipedia.org/wiki/Defamation#Truth http://en.wikipedia.org/wiki/Defamation#Truth). While Yishan's actions may be considered unprofessional, he is legally in the clear so long as he was not lying. I think you may be confusing defamation with disparagement; if a non-disparagement contract existed, then Yishan could be in legal trouble.