"The IRS says you have to mail the money back to a special address."

This was because these initial payments were based on 2018 (and some 2019) tax returns. This was correct: "but she received both the first and the second payments."

2020 is all that matters.

"died in 2020 before the first stimulus payment"

There is no Splitting the year.

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.

https://www.irs.gov/newsroom/economic-impact-payment-information-center-topic-a-eip-eligibility

https://www.irs.gov/coronavirus/second-eip-faqs#Eligibility

One for each EIP.