Adjust Basis in remaining Shares of a large lot?

[Rojo43]

My father died owning a large number of shares which stepped up in basis on his death.  In 2020 my mother sold a about half the shares but erroneously reported the original purchase price as the basis of the sale.   By not reporting the Stepped Up basis she reported a much larger capital gain than she should have.

Now three years later the tax return cannot be corrected.

The question I have is can I adjust the basis of the remaining shares she owns; deducting the basis she reported in 2020 from the date of death basis, for the remaining shares.  

A capital loss would be result.

[Mike9241]

if you asked for a specific rule, I couldn't find one. From a technical standpoint income and deductions can only be reported in the proper year which means that the basis to use for tax purposes would be only that remaining if the original sale had been reported correctly.