I have a different take on this but I'm sure George has his reasons for the position.
If your client did not claim any depreciation on an asset, that, in itself, is an impermissible method. Since your client has used that impermissible method for two or more years (i.e. 2018 and 2019), consistent tax treatment is established and a method of accounting is deemed to have been adopted. To start claiming depreciation on the asset, your client should file a F.3115 for the change of accounting method and the catch-up depreciation will be a negative §481(a) adjustment allowable as an expense on the Sch E of the year of change (presumably 2020 in this case).
Had your client discovered the omission when the 2019 return was prepared, she could have simply filed an amended 2018 return to correct the depreciation prior to filing the 2019 return, which would then show the proper depreciation based on the amended 2018 return.
Still an AllStar