Her IPad is set to send emails from her plusnet mailbox via our Thunderbird client
... above confuses me too.
Whilst Thunderbird may be the client used on windows, I would not expect Thunderbird to have any relevance to iOS, which has its own email client. I think you probably need to simply forget about Thunderbird when configuring iOS, and just configure it as instructed in the links posted by Weaver.
I see the Plusnet instructions in Weaver’s link indicates a default to IMAP, which I’d agree with, use IMAP. However, once iOS mail is working, you might need to revisit the Thunderbird configuration on Windows, to make sure that is also using IMAP, otherwise you may find mail fails to show up the same on both clients. Emphasise though, get iOS working first, before thinking about changing anything on Windows.