Earlier quoted context omitted.
If he's working in the US, then debatably yes. Though 'jurisdiction' isn't the right word. Work performed by an employee during the course of operations is considered 'work for hire', and is the intellectual property of the employer. If he then goes on to replicate a large quantity of that work 'on his own time', he is most likely infringing on that work that he previously did (unless he's very careful about it. Sinc…
What if that's the other way around? He wrote something for himself and at later time was hired and did some work for company and that work was based on his previous work that he had all copyright for. Wouldn't company had no right to use the code that he wrote for them because it would mean infringing on his copyright, or copyright of his mom if he sold the rights to the original software to her?
Personally, I would write that 'previous work' as a library, open-source it, and then use the library at the company later. That's clearly safe.