You never actually report and pay income tax on your 2019 amount of income. Why would you wanna pay more income tax? Your tax return must only list your correct 2020 income.
The function that allows you to calculate earned income credit based on your 2019 income is a separate option in the program. When you run that option, you will be asked to provide information from your 2019 tax return and TurboTax will determine which method provides the greater earned income payment.
if your girlfriend did not run this option in the program, you can test to see if the option improves your situation and then you could file an amended return after your first refund is paid. However you need to be aware, that even though the earned income credit can be calculated based on your 2019 earned income from working, you can be disqualified if you have unearned income in 2020, such as unemployment compensation. So the benefit of the alternative calculation is not as great as some people assume.
