So how the heck can I download Google Earth to his new Mac?
You already have. You get the alert when you try to open the app from (probably) the .zip file. I haven't wanted GE for several years so I haven't tried to download it, but this dialog is common for any/every new app I download. This method been around since at least Sierra. It's part of Apple's SIP.
Not there in any of our Macs sys prefs.
What OS are you using? Thought you had a new Mac and thus had
macOS 10.13 High SierraYou can also allow the computer to simply start up without requiring a password by enabling Automatic login with whoever is designated as an Admin. System Prefs->Users & Groups->Login Options->Automatic login.
I believe the checkbox
Jon is referring to is in System Prefs->Security & Privacy upper section concerning login passwords: "Require password [timeframe] after sleep or screen saver begins".
BTW, a password has nothing to do with "scan [for malware] for it now and then." Scanning, after the fact, is like shutting the barn door after the horse has left."
"We limit the use of our Macs carefully, so never had malware" is also irrelevant to using a strong password on the computer/phone/tablet/car/house/etc. "Malware" has even been discovered in an app hosted by the App Store, it is out there and you may already have it. And, without a password on the Mac, there will be no chance to see when it is trying to change something in your system.
Not wanting to enter a password when the computer sleeps is easy to solve. "Don't throw out the baby with the bath water."