Gmail is definitely the best bet for them.
I have a Fastmail paid account, but their "free" account is really nothing more than a demo at this point; more of a sampler to see if you like it. Too limited to be of much use otherwise.
With Gmail you can even give them separate accounts. And if they need ongoing help and/or monitoring with their email, you can see if they will agree to let you have their password(s) and then set the Gmail account up as IMAP. That way they won't lose messages in error - they'll always be on the server, and you can access them anytime without downloading them from the server, therefore getting them before the couple can.