The problem is that they don't have a backup.It may be simpler to replace the drive or take the drive out of the machine, connect to another computer, erase it, put it back into the Macbook, and reinstall the os.
OP stated macOS High Sierra, but that doesn't fit to Disk Utility image of post #12 that shows DU of pre El Capitan area.2. What version of the OS is on it