Make a live CD Debian with dwagent (https://www.dwservice.net/)

sudo apt install debootstrap xorriso live-buil
7647
d syslinux squashfs-tools isolinux grub-efi-amd64-bin

cp config.dist config

Change livework and os_version to your needs.

sudo ./make_iso.sh

If no error, you should get dwlive-debian-amd64.iso.
Now, you can copy the iso on a usb stick.

«IT support» = the person helping
«client» = the person who needs help

Sign up on https://www.dwservice.net/
Go to Agents. Add new Agent.
Send the code (like 123-456-789) and the dwlive's iso to your client
Wait your client enter the code.
Afer that, you have access to his computer.

If you whant to fix de Gnu/Linux, you can run it with display.
This is how to do.
Mount the OS partition :

mount /dev/sda1 /media/ # change sda1 to your conf

or for lvm

vgscan
vgchange -ay ubuntu-vg # change ubuntu-vg to your conf
mount /dev/ubuntu-vg/ubuntu-lv /media/

Start X server :

./1_startx.sh

Now on https://www.dwservice.net/, you can select a screen.
Go to the screen tab, and

2_run_os.sh

After login, you can do :

export DISPLAY=:1
mate-session& # or cinamon-session or kde-session or...

Copy dwlive on a usb stick.
Boot the computer on the usb stick.
Valide questions about the key board. If you didn't know the answer, just press enter.
At the question «Please choose an operation», select 1.
Then enter the code given by your IT support (like 123-456-789).