Your client will be eligible for the $500 Recovery Rebate Credit with respect to the qualifying child on her 2020 return. Period.
It's in Sec. 6428. Recipient of EIP in excess of the Recovery Rebate Credit will not need to pay back the difference. Recovery Rebate Credit of an eligible taxpayer will be computed based on information on the 2020 return and that will be reconciled against the EIP that particular individual had received.
This is also explained in EIP Q J3: https://www.irs.gov/newsroom/economic-impact-payment-information-center-topic-j-reconciling-on-your-...
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