Make sure they have the Share screen options in settings and restart the computer
sudo apt-get install vino
Then also they will be using IP address to connect to the computer
And the computer has to be connected to ethernet. Used Real VNC Viewer free version to make it work on windows for the linux and to find the IP address use the code below
hostname -I
Sometimes Ubuntu will have an encryption issue use this code in linux
gsettings set org.gnome.Vino require-encryption false