Unless LR have changed, there’s built in tools to move and manage the db <-> file relationship.
There used to also be tools to prune/trim the db.
The root of the problem might her workflow. Especially when shooting series, when she’s back/at the computer, immediately go through the photos and delete whatever’s out of focus/uninteresting/etc.
Maybe she’s good at the workflow? I know how tedious it is to sit down and sort out a huge archive like that.
I used LR for a long time, but got tired of backing up and managing the files. I moved to iCloud and I’m happy with that.
The Apple Photos program is extremely basic, but it’s a decent way of rough organising photos.
Using tags/project/etc. is important.
As for folder structure, I used (year) - (month) (day). That with at least some basic tags made it manageable.
It was (is?) common to also have multiple libraries. Like a temp library, could be used on imports for example. A done library for client jobs completed. A personal library etc.