If he’s not using the computer to surf to suspect sites and not opening up and running .exe progrmas from emails sent frompeople he doesn’t know, then he shouldn’t be having the problems he’s having. Thus, someone is using his computer to go to places they shouldn’t be going or downloading suspect programs.
I have heard that this is the most comprehensive virus software on the market
Tell him and all his employees - no web surfing. These computers help him make his living. They are not toys. If you surf the net - you will get viruses etc. Open up any questionable software - viruses. Use the home machine for play.