The main problem is that usual things are becoming just too complicated for that person. There are too many details that don't matter. For example, user interfaces have a lot of features that are not strictly needed, many hidden states, often no clearly visible affordances, and so on. Some mail clients make it difficult to recognize phishing.

As a good example how it is done better, I'd name GNOME simple-scan. A really good and simple UI!

This person is currently using Thunderbird + Firefox on Windows, and K9 Mail on Android. Learned to use Windows in their sixties and is now in mid-eighties age.

The main need is a simpler Email client.

Any suggestions?