If they are willing and able to learn something with a radically different interface: Fedora.
If they aren’t: Linux Mint. There is nothing easier for a former windows user than Mint and it has a ton of built in tools for any kind of user. Not to mention forums support and massive package repos (+ built in flatpak).
iOS has an official app for Lemmy called Mlem. It’s currently in beta testing but it’s open to any one that downloads TestFlight and accepts the request.