3115 missed depreciation

"Can she claim depreciation in 2019 and then file the 3115 for 2020 to take missed depreciation."

I don't think that is the way it works.  I think you want to do it with the 2019 return.

Are you doing 2019, and 2020 that she did not file or are you amending those two years. If 2019 has not been filed, then like ironman said, you should file the 3115 for that year. If you are amending (because of missed deprecation) then I would just amend 2020, and do the 3115 for 2020.

I'm no expert (I can count on one hand the number of these I've filed) but I thought the 3115 had to be filed with a timely filed return?  Maybe @TaxGuyBill knows.

From the instructions page 2:

Attach the original Form 3115 to the filer's timely filed
(including extensions) federal income tax return for the year of
change.

But maybe this will help?:

Late Application
In general, a filer that fails to timely file a Form 3115 will not be
granted an extension of time to file except in unusual and
compelling circumstances. See section 6.03(4)(b) of Rev. Proc.
2015-13 and Regulations section 301.9100-3 for the standards
that must be met. For information on the period of limitations,
see section 5.03(2) of Rev. Proc. 2023-1.

I agree with Rick - you can't claim ANY depreciation.

The usual rule is that it must be on an original, timely filed return.  For disposed property there is an exception and can be done on an amended return within the three year limit, but that doesn't apply in your situation.

As weird as it sounds, I don't think you can even claim any depreciation for 2019 or 2020 because you are not allowed to without a 3115.

But that makes me wonder ... if you have no way to claim depreciation for 2019 or 2020 (you are not allowed to), then maybe that doesn't lower Basis?  I never thought of it until now, but that would be interesting to research.