I agree with Tagteam. Form 3115 can not be used for an incorrect Basis (a "mathematical or posting error").
Use the correct Basis on the current return. Leave the "prior depreciation" either BLANK or enter the amount of depreciation that SHOULD have been taken.
You may want to amend 2018, 2019 and 2020 to claim the proper depreciation and possibly get an additional refund for the extra depreciation deduction. But as you mentioned, if she still owns the home when she dies, all of the depreciation disappears so it would not affect anything when it is sold (based on current tax law).
