KDE

My experience is that it's similar enough out of the box that it's fine for more users. Unless she's really disturbed by anything that doesn't look absolutely identical, having some noticeable differences is good as it's a bit of a reminder that you're not on Windows but works similarly to Windows.

Like, you could go all on and install a Windows 7 or 10 theme and make it look like Windows nearly pixel perfect, but then you set the expectation that it works exactly like Windows and that leads to disappointment as soon as the illusion falls apart, and she also won't learn to explore new features if it's presented as diet Windows. You want it to look familiar, not sell a lie or an illusion.

I got my mom to use Linux for a while, it was hard to get her to move on from XP to 7 and then 7 to 10. The switch to Linux went comparatively smoothly, because it was clear from the start it's not Windows, it doesn't work like Windows. But it retained the familiarity of the start menu, the task bar, the tray icons, so she felt comfortable using it and exploring it but came in with an open mind and without the expectation and frustration that came with trying to do something and realizing it's not there. She knows she won't just download a .exe and run it, that she has to use Discover.