This is a good example of the limitations of Radical Reference to answer time-sensitive questions. We did send a few suggestions the next day, but the situation described involves not only some tricky human-relations questions, but legal ones as well.
This gets us very quickly to the "slippery slope" of legal advice, which (like medical advice) we librarians can't give. Possible research before contacting a labor relations service or government mediation agency, or attorney, etc., might include organizations that serve the work you are in, and of course any human-relations people or mediators that you trust in your own organization. This would help document a good faith effort on your part to resolve the problem. For example, if you work for a non profit, there are organizations of non profits in your region that may have articles or information online. One example is the Virginia Network of Non-Profits. In your state government's site (use the pattern www.virginia.gov), a site search such as: "labor relations" or even "conflict resolution" might lead you to agencies that could help. But it might take plowing through many job ads, biographies of agency heads, and many other irrelevant sites. It would take quite a bit longer, but most public libraries give access to magazine full-text databases such as Masterfile, where you could do searches such as: firing and conflict resolution; employee termination and best practices, etc.
Other Rad Ref librarians suggest negotiating untaken vacation or sick time with the employee if possible; and checking the Department of Labor site, where the search "termination of employment" gets a page on termination issues The Fair Labor Standards Act website includes text of the law + regulations.
But you will very likely want to keep a written record of what you do - a paper trail to try to avoid further grief from an unpleasant situation