Who is the licensed owner of QBs? I'm assuming it is the Clients.
They just need to open QBs, go to the Company drop-down menu, My Company. When the window opens look in the upper right corner. If the old Bookkeeper is logged in, just do the drop-down next to the name and Sign Out. Then sign in with their credentials.
If for some reason they do not have their own version of QBs, they need to get one. But the Company File will still work. Just purchase QBs, install it and set up their own account. During the installation, just choose Open Existing Company and go to the folder that has the Company file.
NOTE: They should make a copy of the folder that contains the Company file (on the desktop or wherever) before they do the installation. And they may have to purchase QB2022 if they can't find QB2021, so it is okay when QBs asks to update the file When you go to open the Company file, be sure to to go the original file, not the copy you put on the Desktop.