Amending the 2019 tax return will have no effect on any stimulus she was eligible to receive.
If she was not a dependent in 2020 and otherwise eligible, she should have filed a 2020 tax return for any stimulus payments she did not receive, either the first or second payment. Payments not received are entered on the 2020 tax return as a Recovery Rebate Credit on Form 1040 Line 30. The credit is included in the 2020 federal tax refund or the credit will reduce any 2020 federal taxes owed.
The third stimulus payment is sent by the IRS based on the latest federal tax return on file with the IRS, either the 2020 or 2019 tax return.
Go to this IRS website for information on the third stimulus payment - https://www.irs.gov/coronavirus/get-my-payment
