@Arble H Gimbleh ,
Now I understand your predicament --- you need all the transcripts of the returns filed ( as far as you can get ) and nay and all information filings ( W-2, 1099s etc. ), in an effort to bring the filer into compliance.
(a) Since you already have a valid POA and assuming that you have the personal details of the taxpayer ( such as SSN etc. ), you can always go look at her personal account ( or set one up ) and see what is there with the IRS --- see this page --> Transcript types for individuals and ways to order them | Internal Revenue Service
(b) you can also file form 4506-T for all the tax years that you need or you can visit the local IRS office with the POA in hand and seek guidance.
I am saying the above because the taxpayer is alive, just not able to do the needful herself but you can set up her personal; account ( if she has not done so herself) based on the valid POA.
Once you have the details, I would suggest starting with the latest years and work backwards ( since you have the TurboTax software for the needed years. Alsop note that while IRS ( and many states ) will allow refunds only for three years prior, the tax liability for all the earlier years ( with interest and penalty ) can go as far back as possible ( at least six years for sure).
Also if she has not received any notices from the IRS ( that you are aware of ), then she either never filed or the informational filings were too small for the IRS to chase/ send notice to non-filing. It is a difficult situation .
Good Luck
