It seems they were for tax purposes legally married- I'll assume they lived apart after the "divorce". if not they could only file jointly or married filing separately.
2015,2016,2017,2018,2019
for these years if they did not live apart, they would have to recompute their liability either joint or married filing separately. if they don't owe then nothing needs to be done since the statute of limitations for these years have passed. if they owe they need to file amended return because the IRS can after them forever. if they lived apart for the last six months of the tax year and one paid more than 1/2 the cost of maintaining a home for themself and their child who lived with him/her for more than six months he/she could properly file as head of household. but the other not meeting the condition specified (maintaining household for child) would have to file as married filing separately not as single. this person would have to recompute their liability for these years and if they owe file amended returns.
it extremely doubtful that any additional refunds would be due the taxpayers, but if they were they're lost due to the statute of limitations.
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basically, the same rules for 2020, 2021 and 2022 except if they are due additional refunds, they can file to claim them.
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code sections 1,2, 6013, 7703 which have the rules for what filing status to use.
