In a round about way to fix this, you can do the following
Temporarily add another drive to PC
Or what i often do use gparted and partition the bad disk.
If you have no skills in this, use the additional disk.
As in all things in life make a backup of your damaged disk just incase it goes further down the bad way. Use clonezilla or dfsee or any of your fav tools.
Use an install disk, Use an iso of 1709, 32 or 64 bit doesnt matter.
Install a new instance of win10 in the new location (leaving the first drive - the bad one - as the primary.)
If you have not bastardised the boot partition too much (and renamed it back to boot) you will have a new bootable env
Maybe even having rediscovered original win install and added it to the boot menu.
Now if you think you totally buggered the original boot directory, just format it first - using gparted -
or let the new install just do it.
If you have a booting system (going to the new installed env) it will be ‘C’ drive and the other disk/partition will be identified with ‘D’. So now you can navigate around that and see what you have done to it.
If you have both envs in the boot menu when you boot the other one. They are automatically again identified correctly.
So you have a boot partition on the original drive and its running os and another running os in the other drive.
To get the PC back to your client’s state is very simple, just remove the the other disk. And when you boot up the PC WIN10 automatically adjusts the menu, as it is one again the only entry, the bootmenu is not presented
The $5 reward is a bit rude as this idiots work takes ~1hour. But is very simple and controlled to execute.
I wont give you powershell scripts ro rebuild, as i dont think you are ready tomsafely use them.