Disaster that should have been caught earlier

Of course, hindsight is always 20/20....😄

But...I may have done the same things, in a slightly different order.

1. Disconnect Time Machine backup drive.
2. Boot into Safe Mode and see if anything changed.
3. Assuming (2) didn't change anything, I'd want to know if third party software is interfering with things. Maybe run Etrecheck and see if any "nasties" like anti-virus, Mac cleaners, and the like are installed.
4. Assuming (2) didn't change anything, and Etrecheck didn't reveal anything, boot into Recovery Mode and reinstall the MacOS...which would not delete any user data or settings.

It sounds as if the issue that caused the Finder malfunction was never figured out?

Regarding the excellent backup advice BobTheFisherman has provided....it's 100% correct. The only thing it doesn't consider is the convenience factor. For that reason I'd also add online data storage services like BackBlaze or iDrive as backup options.

The_Knowledge_Seeker wrote:

My question is: what could she have done differently (apart from fixing the Time Machine drive a month earlier when it failed).

It is standard practice for computer users who value their data to use the 3-2-1 backup strategy which simply states that you should have 3 copies of your data (your production data and 2 backup copies) on two different media with one copy off-site for disaster recovery.

I agree with everything BobTheFisherman wrote. I would also ask her to consider getting a second Time Machine drive and run both together if your colleague feels a lot of anxiety.