Since the debt was cancelled in 2024 in a year after she was deceased, the income it represents did not belong on her final tax return. Thus, if you filed a final tax return for 2023 you don't need to amend it.
The cancelled debt in 2024 would need to be entered on an estate tax return for her if it was legitimate. However, I assume such an estate does not exist and if it did, the cancelled debt income could be exempted as the estate would be bankrupt since it has no assets.
For practical purposes, if the estate does not have an executor, there is no one the IRS could turn to to require a return to be filed so it is not likely the IRS would pursue the matter.
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