Accidentally merged one iCloud Photos library with another -- how to revert back?

Here's the tick-tock of how this happened: - Daughter A had an old iPad that she rarely used, so she offered to give it to Daughter B. - Daughter A signed out of her iCloud account on the iPad, and (when prompted) indicated that she wanted to remove all of her personal info (Contacts, Calendar, etc.) from the iPad. *However*, apparently doing this does *not* remove photos that are locally stored on the iPad... something that she did not realize and did not check before handing the iPad over to its new owner. - Daughter B signed into her iCloud account on the iPad. Later that day she opened the Photos app and saw not only her own photos, but also all of Daughter A's photos. Whoops! - Daughter B then looked on her phone and saw that there, too, all of Daughter A's photos were mixed in with her own. Is there any way to disentangle these two photo libraries? Can I revert Daughter B's library back to the state it was in 24 or 48 hours ago? (Searching online for "revert Photos library" brings up dozens of sites about restoring deleted photos, which is not the problem I'm having.)