"She is now claiming their daughter"
And that's all that matters.
Perhaps it would help to review what is really happening:
The funds were paid out as Advanced payment against a projection. The projection used 2018 or 2019 tax returns. But 2020 is the Actuals. You use the 2020 return to reconcile what a person is entitled to, against what they got.
You might want to bookmark these links and read the IRS guidance.
Interactive wizards portal for determining dependency:
One for each EIP.
"Level Up" is a gaming function, not a real life function.