Symptoms:
When you launch QuickBooks, it throws below error message,

"Error -6123, 0: Connection to the QuickBooks company file has been lost"

Cause:
This happens only if QuickBooks you are trying to access is installed in a remote location and it could be any of the following reason due to which above mentioned error message pops-up while launching QuickBooks. A network connection with the host computer is disrupted. Firewall settings prevent QuickBooks files from accessing the necessary ports. QuickBooks data or program files are damaged.

Resolution:
Suggest them to contact local IT help desk to find out if the network connection between the host and the remote server on which QuickBooks is installed is up and running and make sure there is no firewall settings prevents accessing the company file, if so, ask their IT team to  create an exception to access QuickBooks company files or else Quick Books program files could have been damaged.