Another vote for Thunderbird
When asked, I almost always recommend Thunderbird, regardless of age but a couple of elderly neighbours found it very straightforward and picked it up quite easily. In both cases, I did take the time to work through it with them, concentrating on the basics, read, write, send, handle attachments, etc. Thunderbird is free from Mozilla (introduce them to Firefox too, while you are there) and has no ads or other screen clutter.
One big advantage of a POP client, rather than a web based mail system is that if the user doesn't have a broadband connection, it will keep the dial-up phone bills down by allowing offline reading and composing, just going online briefly for a send and receive of pending messages.