Mom's taxes are past due. How do you handle late, complicated taxes?

Yes, you need to go to a Certified CPA. You have, if you have a doctor's letter, good evidence of Mom's inability to file taxes (UNLESS she had a POA responsible to do so). If there were taxes owing there will be penalities and etc that will make the taxes themselves seem lightweight. You should do this at once, taking all the information you are able to. The CPA will guide you in how to reach out to the IRS.

I would check with a good tax preparer, one with a CPA or at least accting background, if Mom even needs to file taxes. (No H&R block places)

It all depends on Moms monthly income. If that's only Social Security and maybe a small pension, than she may be under the cap set. My Mom made 17k in SS a month and $200 in a sm pension. That made her a little over 20k a year mostly SS. She received a letter from IRS saying she no longer needed to file taxes.

Here's what I found. Just giving u an idea. You still need to talk to a tax preparer.

When seniors must file
For tax year 2020, you will need to file a return if:
you are unmarried,
at least 65 years of age, and
your gross income is $14,050 or more.
However, if you live on Social Security benefits alone, you don't include this in gross income. If this is the only income you receive, then your gross income equals zero, and you don't have to file a federal income tax return.
But if you do earn other income that is not tax-exempt, then each year you must determine whether the total exceeds $14,050.

Your profile says your mom is 84 years old with dementia/ALZ. Just putting it on here so that there is more context to your question so you get more appropriate responses.

Also, can you add more detail about what you think makes it "complicated"? Thanks.