By releasing the code under a permissive license, he gave up that recourse. His client's right to use the code is now a grey area. Technically, he has the same recourse in court as he did before; contracts. But he's severed the connection between that and the deployment of his code.
I'm not expressing nerdly outrage at how he did it. I'm simply suggesting a tactical refinement for the next person who decides to dump dox to Github when a client doesn't pay.
Not in China.