I would do just the parent, there’s no point in doing the individual dirs.
But I very much question why you would use FFS to sync data through a router with god knows how high quality firmware for its disk/file sharing, and then over a potentially shaky USB connection to a disk which by now is the third thing that can fail along this way, and then in order to back this up have to read the data back over this infrastructure, before sending it off using restic.
A much better solution if you ask me is to just put restic on her computer and back up straight from there to B2. If you still want the USB disk to have a copy as well, then either use restic to back up to that one too, or keep using FFS as a secondary backup (it’s never a bad idea to have multiple backup softwares).