I doubt it will be "copyright infringement" since the code was rewritten. It might fall under patent infringment if something was patented.

The more likely scenario is that it will constitute a misuse of trade secret either under law or from the contract signed on employment. I am not a lawyer and the law is different in every place but it is likely that such a use would be protected under paw by protections given to professionals to retain expertease gained while practicing their professions. I don't remember how that protection is called.