Linux

Out of the box, KDE has the most similar look and feel as Windows. That said, I do believe Linux Mint has Windows themed DEs that go one step further

Linux mint would be the best for windows user

Go Kubuntu if you want Ubuntu and Windows-like. Otherwise, Mint is a solid choice for a Windows user too, I believe Mate and Cinnamon both are very similar UI to Windows. Sadly Mint only does LTS versions now, but maybe that's for the best if your GF isn't super into tech?

Why do you want to get your girlfriend mad at you?

I'd say KDE since it offers fully working Wayland already.

My GF is happy with Linux Mint Cinnamon! Even my parents use it :)

This is my Linux Mint Cinnamon dressed up as Windows XP. How much more Windowsy would you like it ;-)

I'd give her Linux Mint with Cinnamon/MATE instead.

My dad also made the switch to mint cinnamon about 3 years ago and I only had to fix things once for him (which was something in partitioning/fstab he or the installer messed up), he has successfully updated and maintained the system for 3 major releases yet and is even happier with Linux on his home laptop than with windows on his work laptop

Edit: he's not really tech savvy or something, he's a teacher by profession

KDE is more similar to windows, Gnome is maybe closer to MacOS.

But generally, KDE felt always a bit "super user" to me, and I found the amount of customizability and features more overwhelming than useful. Gnome is simple, clean, and I prefer it for that reason.

Linux Mint is the way to go. It's by no means a clone of Windows(yuk) but has a "start button", vertical menu with apps listed and a bottom taskbar. So it should be very easy to use and get used to.

I'de rather go with Linux Mint Cinnamon or Nobara with their custom GNOME DE

KDE plasma and cinnamon

While Gnome can be shaped into something Windowslike with extensions 'Arcmenu' and 'Dash to Panel', for another user I'd suggest just using a DE that already works like that. Best choices IMO are Cinnamon or KDE.

Since you already want to use a Ubuntu desktop, I'll stick to suggesting Ubuntu flavours. KDE on Ubuntu, you'll want Kubuntu. For Cinnamon, there is a Ubuntu Cinnamon flavour.

KDE is somewhat more modern-feeling than Cinnamon but also uses more resources. If your GF's system is a bit weak, maybe go for Cinnamon. Cinnamon is also a bit retro in it's design, so it might be more comfortable for people used to older Windows versions.

I recommend downloading a live installer for both, showing her them and letting her pick the one she likes the most. (On that note, I hope you got her permission to do this...)

Plasma/KDE

Well cinnamon and plasma are good options, those de are widely available. Zorin os was a quick select option that lets you go from windows 7 to windows 10 to mac with a wimple click.

If you must use ubuntu, you'll ultimately be tied to the desktop environments that ubuntu offers. Personally I don't think ubuntu works well, especially outside of their gnome offshoot Unity DE. If you must use ubuntu I would say install kubuntu which has KDE Plasma on it. KDE Plasma is highly customizable, quite pretty and pretty easy to use.

KDE Neon or Linux Mint will be your best betnfor a Windows-style layout. Just keep in mind it still isn't Windows so stuff will he different

There are minor distros deliberately styled to look like various versions of Windows. Someone already mentioned Zorin; I'll add Q4OS. Even if you don't want to support one of them in the long term, it might be worth picking up some live media and poking around to see what they did, depending on how much your GF depends on everything on the system being just exactly as she expects.

I'd use mint instead

Check this distro in a VM first https://www.linuxfx.org

Cinnamon has an official Ubuntu flavor now. I would recommend that over Mint, since mint is LTS-only. You can also do KDE-Plasma, but my personal preference is Cinnamon.