I believe that IRS said they have to pay it back. However I have seen several sources that say there was nothing in the original legislation that authorized the IRS collection of the overpayments. (Hoping I used the correct wording,)

There are articles quoting credible sources, including a former IRS Taxpayer Advocate, that claim there is no return requirement. 

She is still filing a Joint return for 2020, I see no reason that she would not qualify for the Credit of the second $600, I would file it that way and see what happens.