I agree to disagree it’s really not that different from vanilla especially if he does go the playstore route, so a person familiar with android can naviagate grapheneos. The only problem would be if they break their device and the thumbprint scanner would have to be recalibrated. Personally I would just keep it as the stock grapheneos setup with accessibility features turned to max. That way its a dumb smartphone
Edit: forgot to mention regardless of os add some kind of network firewall or DNS solution to block malicious/phishing domains. The elderly are often victims of scams