Your options are to file as is, keeping in mind that whether you used the depreciation or not, it will be recaptured at a point of sale, if applicable. Do not change anything without using Form 3115, determine the amount of depreciation that should have been used and deduct the full amount not previously used in the current tax return. The IRS rules, as indicated by our awesome Tax Expert @Cynthiad66, is allowed or allowable.
As far as inheritance goes, the cost basis to the beneficiaries will be the fair market value (FMV) on the date of death. This will have no affect on the beneficiaries, as indicated by our awesome Tax Expert @KrisD15.
The depreciation expense is important to your mom right now and for this reason you should consider completing Form 3115, and if you are unable to do that you should seek a professional to assist you.
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