Can she be set up on her desktop to access the Thunderbird data files residing on my PC from Thunderbird on her system? Can that happen concurrently, i.e. while Thunderbird ios open on my desktop?

That's not possible. But you can set up the account as IMAP on both of your computers. Then each of you can access the account independent from each other, and always see the same messages (on the server).

Hi Christ1, Thanks for the reply. My problem is that IMAP is a problem due to the total size of my data (68 GB) which has been accumulated over the last 23 years... That is why a shared setup would be ideal - we both often have to refer back to old mails. jjph.

If the better part of the 68 GB of data is archived anyway, I'd simply copy over it to the other PC. It doesn't change anymore, so you always access the same data. And this would have the benefit of having another backup copy. I'd hope you do have a backup anyway.

For current (not yet archived) mail you may still go for the IMAP route.