Filed taxes jointly when unmarried?

Hi. My parents have filed jointly the taxes in the past even though they are not married. My mother does not make any income. My parents have lived for several years as a cohabitating couple in California. Was the taxes done correctly? Or should,my father amend and claim my mother as dependent or him as head of... show more Hi. My parents have filed jointly the taxes in the past even though they are not married. My mother does not make any income. My parents have lived for several years as a cohabitating couple in California. Was the taxes done correctly? Or should,my father amend and claim my mother as dependent or him as head of household? What IRS form should I use to make the change? Thank you.
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