Paper submission with company affiliation
If I am understanding this correctly, the paper was submitted while the person was still working for the company.  If this is the case, then the affiliation should stay the same as it was at the time of submission, as it sounds as though the work was explicitly done as part of their responsibilities at the company.  A second affiliation might later be added, for the person's new position, if it is appropriate and the conference supports this, but the first should remain.

If the paper was submitted after the person was fired, that is a much bigger problem, and it is unclear from what you have written whether that would be appropriate, ethical, or even legal.