Linux has come a long way. It used to be complicated, difficult to install and cumbersome to operate. Those were the early days. Times have changed. Linux is now so simple that anyone can install it and use it, even Grandma.
This is assuming you install it for her and Grandma is only browsing the web or checking email and she never automatically updates her distro. That’s right, as long as Grandma doesn’t want to actually do anything with her operating system, everything will be smooth sailing.