Try again with Ubuntu’s Gnome if you’ll have an opportunity. The “problem” with KDE is that it tries to look like and behave similar to Windows, but it’s not, so older people that remembered how to deal with one UI are confused when what looks similar in reality behaves differently. And it results in frustration.

Gnome, on the other hand, provides a totally different UI, so user immediately identifies that it is different and needs to be learned a bit. But thanks to Gnome being pretty coherent and simple in how UI works, it usually takes very little time to learn and then they just keep using it. I experimented with my parents, father is 70, mother 65, and they both earned default Ubuntu very quickly and don’t have any issues using it, unlike win10+, which constantly raised questions and frustrations that something changed (MS likes to bring idiotic widgets to panels and menus after updates no matter that nobody asked for them).