I've found that you retain the important stuff, and unfortunately the embarrassing stuff.
Memory aids are a thing. So if a picture, song, smell, or object helps trigger a fond memory, then keep that thing around. Failing that, writing things down can help unlock the rest of a memory.
Music, and scent are the two main memory triggers though. If you can link a memory to one of those things then you're golden.
I'm glad for you, and I'm happy to use math in every day life, but for me its work.
Many games have this problem for me, but take any game with items that have numerical stats as the only way to differentiate them. So to compare two items the player has to look at the numbers. To me that moment is work, not fun. I prefer a graphical or audio representation of the difference. Maybe color or size indicates the gameplay difference. Something like that makes it more enjoyable to me.
The computer's job is to do math, not mine as a player. Any game that exposes it's math to the extent that a player is incentivized to do math is a failure of game design.
This really depends on her hardware specs and what applications she needs to use.
Without knowing any of that, I would suggest Linux Mint. It is desktop user focussed and a good general OS. It includes drivers and common software in their version of an app store.
Debian is my distro of choice, but is not ideal for a new Linux user.
I would suggest checking what apps she needs and making sure they are available on Linux, or that a close equivalent is. Any apps that will be replaced, try the replacement out on Windows first if available. For example Adobe Illustrator to Inkscape, or MS Office to Libre Office.
For data transfer:
As others have said. Backup the current computer fully. This in probably best done on an external hard drive. Make sure you know how to reinstall windows and restore from the backup.
Copy all her data onto a different external hard drive. This is not the backup. It is a separate drive.
Make sure all the data is actually on the external hard drive and readable from a different machine. Ideally boot from a Lunx live USB and check that the data can be accessed from the external drive.
Install her distro of choice.
Copy her data from the external HDD to her user account's home folder of newly installed Linux.
I have a terrible memory for what year events in the past took place.
Luckily I keep all my old resumes! So I just found out that was the year I went back to school to study music, after having dropped out of high school and working random jobs for the previous 5 years.
I quit a boring office job to work part time in a comic shop at the same time as studying.
Music school was great until a minor drunken injury cause me to defer my studies until the following year.
Walk up and down the stairs instead of taking the elevator at my office (4 floors)
Walk parts my commute where I could take the bus or an extra train. (About an hour walking per day).
I wish I did more but my aging body is too tired 🙃