Genuine question, why is it hard?

Here is a small list of instances if you two want to look into it some more. :) I wouldn’t recommend joining bookwyrm.social though, as it’s the biggest one and can be a bit slow sometimes. Also, it’s more in the spirit of the fediverse to spread out over several smaller instances. :D

She wouldn’t need to host her own instance. Like for Lemmy, kbin or Mastodon there are plenty of Bokwyrm instances with open registrations for everyone.
And they all federate with each other and the rest of the fediverse. Which is also good, because books have to be added to the instances manually, but thanks to federation there are already many books added that also federeate between instances (sometimes you just have to import the full book data from elsewhere with one click). Only if it’s a very niche book you might have to add it completely, which is fun though imo, because it helps Bookwyrm grow.
And there is the possibility to import your reading data from Goodreads. :)