A person can NOT file as single if they are “considered unmarried.“ The fact that you did not live together would only qualify the ex for Head of Household if they had qualifying children. The accountant is wrong if you did not have a "legal separation" agreement in place with the courts. Stand your ground ... you did nothing wrong. Read the IRS publication 501 under the FILING STATUS section for the rules. https://www.irs.gov/forms-pubs/about-publication-501
