Get your taxes done using TurboTax

Understand this --- she cannot get tax refunds for any tax years earlier than 2019, but if she owes tax for any of the tax years in which she did not file, she still has to pay the tax due.   Any refunds from 2018 or earlier have been forfeited.   Tax "due" never goes away, and could be owed with penalties and interest.

You say that she thinks she will get refunds for the past several years because she had medical expenses.   Refunds come from paying in more tax than you owe.   Her medical expenses may not generate refunds.   it is very difficult to get a refund based on medical expenses.    Only the amount that is over 7.5% of your adjusted gross income can be used as a medical deduction.   If she has not paid tax or had tax withheld from any income, there is nothing from which to issue a refund.

You say that she receives Social Security.   Did she receive the stimulus checks?   Recipients of SS were supposed to receive those automatically, so she should have received the 1st one of $1200, the 2nd one of $600, and the 3rd one of $1400.   If she did not receive them, she can get them on tax returns for 2020 and 2021 by using the recovery rebate credit.

Software is available for the 2018 onward, but the online software is only for the current tax year of 2021.   Any earlier years must be prepared using download desktop software on a full PC or Mac.   Only a 2021 return can be e-filed.   Other years can only be filed by mail, which takes a very long time (6-9 months) to be processed.  Other years would have to be prepared by hand, using the paper forms.

You say she has been receiving Social Security and a pension, as well as rental income.   It would be better for her to seek help from a paid tax professional in your area.   She may need help in gathering together all of the records she will need to provide.    This does not sounds like a good do-it -yourself project.