Filed Single by accident

She will file an amended return, change her status from single to joint, and add her spouse's information, income, deductions and so on.  The spouse will not file anything on their own.

https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-current-year/L7eS6o1qh_US_en_US

I'm not an expert; be sure to read the instructions for Amended Returns before filing again.

But based on decades of doing complex returns, my sense is that once filed, a return can't simply be undone. You must instead file an amended return which corrects anything that needs correcting. Plus, you must fill out another form (1040-X I believe) that summarizes the changes from the original to the amended return.

If your daughter files as Married Filing Separately, I'd guess that there would be changes mostly to the standard deduction (unless her return is complex). But those changes would impact (and increase) the tax due.

If she will need more time to prepare the revised return, be sure to file a request for extension with both the feds & state (if your state has an income tax system) by the 15th, and pay a chunk more than what was calculated already. That will avoid interest & penalties were she to pay too little by the 15th.

"I forgot I was married ?"

If she got married this year (2025) then she cannot file MFJ for 2024 since she was not married then. Next year she can file MFJ for 2025 taxes.