Whether an employee quits or is fired, you will need to handle certain basic paperwork and legal requirements. For example, you'll have to cut the employee a final paycheck within the time limits set by your state's law. You'll need to get back company property, deactivate passwords and access codes, and so on. And, you'll need to provide paperwork on continuing health benefits, if the employee is entitled to COBRA continuation coverage. For a checklist of practical matters you'll need to handle for departing employees, see Nolo's article When an Employee Leaves.
For legal advice, you'll need to talk to a lawyer.