Everyone hypes Mint but if you're working with newish hardware you might have a bad time due to the drivers taking a while to mature and filter down through all the distros. If her rig is a couple years old it should work just fine though. I would also suggest trying out Kubuntu, Pop!_OS, PikaOS, and Zorin if that is the case.

If she is on brand new hardware then something Arch based is the way to go IMO. CachyOS, Garuda, and EndeavorOS are all Arch based distros that make setup easy and they've all worked great for me out of the box. Honestly if you have snapshots configured with timeshift or something being on a rolling distro isn't as scary as it's made out to be. Fedora is an option too as they get updates every 6 months, but there is a little extra setup to do after install like media codecs and proprietary drivers etc.

Cachyos was my personal pick and it's working perfect for me so far.

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:

1. 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.

2. Copy all her data onto a different external hard drive. This is not the backup. It is a separate drive.

3. 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.

4. Install her distro of choice.

5. Copy her data from the external HDD to her user account's home folder of newly installed Linux.

Well quite obvious: as the name "Debian" was coined to celebrate the union between Debra and Ian, makes it a de facto choice! ;)

I would recommend either Debian or Devuan - both are absolutely rock stable and are a good entry level drug for the Debian based ecosystem. I personally like Devuan more (it just feels more mature and has more ~~old~~ mature community members).

Debian or Ubuntu because they're stable and well-funded. Makes a lot of stuff easier.

I think Linux Mint would be a good first distro.

I recently learned about a project called Operese. It is a Windows to Linux migration tool that also sets up Kubuntu. Kubuntu is Ubuntu with the KDE desktop environment instead of the GNOME desktop environment. I don't know how well that tool works since I never tried it but it looks promising.

There is also a new project called Winboat that is meant to make it easier to install and use Windows software such as Adobe Photoshop

I loved Mint. It's still great. Recently I installed Linux on a family member's laptop which is not upgradeable to Windows 11. Hate to say it (and I may be a bit petty here): Mint looks dated, Cinnamon needs a facelift.

That was a reason I went with Zorin. It clearly tries to transition users that come from Windows with it's design (honestly, it's modded Gnome looks awesome). Even running .exe files is as simple as just opening them. Zorin will either just run them or suggest a Linux alternative. Had no issues with that OS so far.

That said, Mint or Ubuntu are solid choices for beginners (and pros alike).

Put Linux on one of those spare hard drives and simply mount the existing drive as a second drive in Linux.

This will give you access to all your current files from within Linux without having to do anything. Move over what you want and need as you use Linux. At some point, you will probably want to reformat the original Windows drive for extra space. You could consider mounting it as /home at that point.

Choosing a distro is a matter of taste. I can tell you though that I have moved a few Windows users to Linux Mint and they are all happy with it. My last one was LMDE (Mint with a Debian base).

Bazzite

Mint Cinnamon. Just make sure to change the background before she sees it. The first impression is god awful with that stock background.

I think basically all the default backgrounds aren't great. There are a few passable ones but that's it.

Opensuse tumbleweed. It's solid, stays up to date because it's a rolling release, and Yast can be a life saver for new users.

Debian + KDE