

Mint.
It’s extremely stable Linux for your grandma, that comes with every tool that she will ever use and on the cinnamon interface all those tools are exactly where she will expect them to be if she is used to using Windows.
I’ve gotten three boomers to use it and they hardly ever ask for tech support because it’s so stable.
I know a guy who worked on Unix in the '80s and he is very clear that Linux / MacOS are just Unix.