If the defendant in a case once worked for the prosecuting attorney is there a conflict of interest

Much more information would be needed to determine whether there is a conflict of interest. You likely can contact the agency that licenses and regulates attorneys in the relevant state to ask.

In general, the attorney would have a conflict of interest if the attorney learnt some confidential information about the party because of an attorney-client relationship with the party.

Merely because the attorney and a party once had an employer-employee relationship likely would not be enough to preclude the attorney from representing another party whose interest is adverse to the former employee.