There is Goldfish, which was started with beeing some sort of TikTok alternative in mind. But it's stuck in a very early stage and doesn't seem to be developed anymore. If I remember correctly the Admin started it as a practice for themself to get more into Activity Pub.
They have a Mastodon account and a github page, maybe you can ask them there if they plan to develop it further. And since it is open source and can be forked by anyone and everyone can contribute to it, maybe someone will be found who is interested in continuing to work on the project.
On Goldfish's github page I also found Vidzy by accident, where developement seems to be more active. However, I found no active instances so far where one could test it.
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. :)