Until she passed away, I had my Mom running Kubuntu for 15 years. The trick is not to match the Windows interface exactly (which has its own issues anyway) but to have an interface that is close enough and pleasant enough to use that learning it is easy. The second most important thing is to pick a distribution and window manager that DOESN'T CHANGE THE UI willy-nilly with every new major release. Even Microsoft makes that mistake on occasion. Gnome is a disaster on that front, and even KDE has stumbled in the past.
What older, less-savvy computer users tend to do is learn procedures and menu locations. Something like "click on this box at the lower-left; go to the third menu down; move right and down and click on the red fox; ah-ha there's the Google search bar (set as the home page in Firefox)". Change the menu position, the icons, whatnot, and you WILL be getting service calls.
I had my Mom running KDE because that's what I use on my Gentoo box, which made support easier for me. (I had her on Gentoo once for a little while long ago--BIG MISTAKE. Don't set up a rolling distribution for non-savvy computer users.) The odds are you'll end up doing support now and then anyway, so pick something YOU can live with as well. My favorite plug-and-play distribution today for beginners is Mint Cinnamon. Mint is based on stable Debian/Ubuntu, and Cinnamon is a very nice and clean desktop interface with a lot of the Windows paradigms, great support, and very little to fuss with. Check out the link provided by a_horse_with_no_name; it recommends it as well and includes a screenshot.