It is possible. She has 3 years to file a 2020 tax return to claim a refund of any taxes paid or the recovery rebate. You should look at your tax return to see if you actually claimed her or not. She would be listed as a dependent on page 1. You wouldn't have received any stimulus money for her, but you would have received the $500 Other Dependent tax credit and she may have qualified you to claim the Head of Household status (if you were otherwise filing Single without her).
But you could certainly remove her as your dependent after your tax return is processed and you receive any refund from the return you already filed. You would need to pay back the tax benefits (from credits or filing status) you received for her though. And after that she could file her own 2020 tax return.
If she received either payment, she doesn't have to pay that back if you claimed her. And, if she received the first one and then files a return after you remove her from your return, she would only get the second payment. In that case, it probably will not be a net gain for your family. You have to take into consideration what you gained (federal and state) by claiming her vs what she gains by going through the process described above.
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