Firings like this often include a technically voluntary separation agreement that gives you a few extra weeks' pay or some additional months of health benefits etc. precisely to avoid that problem. (Also gets them out of paying unemployment, and means they can get a fresh set of NDAs/nondisparagement etc. signed with the employee.)

I would never fire an employee unilaterally, especially over something like this, when there's valuable IP at stake and you can just talk the person into agreeing to sign over whatever it is you need.