This is tricky. A person can file as single if they are “considered unmarried.“ In theory, certain legal separations might qualify to be “considered unmarried“ depending on the laws of the particular state. In practice, I have never actually seen a state law that was regarded by the IRS as valid for this purpose, and I am aware of 2 Tax Court cases in New York and New Jersey that specifically disallow this type of being “considered unmarried“ based on the specific laws of New York and New Jersey.
Her accountant is mistaken in at least one regard; even if you amended your tax return, it will never update the IRS database in time to allow your ex to e-file. She will always have to print her tax return and file by mail.
I would not file as single in your case unless you have a qualified legal opinion that you are “considered unmarried” according to the laws of the state where your separation decree was signed or approved by the court. You need to do what is legal for you according to your own best judgment or the judgment of your legal advisers, and your ex-wife can take care of her own problems.
