I am not sure the "guard staring at monitors" scenario really still exists. Live Monitoring is typically event driven these days, where events can be alerts from an access control system, motion on cameras, analytics, etc. Guards are not "sitting there" just staring at monitors, events are put in front of them (via VMS or automation software).
A key to making live monitoring work is that it needs to be a full-time job for the operator. To add complexity, you are not likely to have events 100% of the time, so there needs to be other tasks for the operator that can be easily interrupted/paused when an event comes in.
For smaller operations, it is difficult to strike this balance, which is why the remote live monitoring operations are becoming more common. They can staff up enough, and fill operators time with other tasks, to make the model more workable.
Is your customer debating hiring their own person that would be on-site in some way, or hiring a remote monitoring company?