Do you have this Security Employee?

"This fella" "Justin" exists – but he's not an Apple Security employee and his real name probably isn't "Justin". He is a criminal, the claim that your father's computer was "hacked" is a lie, and the message is a scam.

Federal Trade Commission – How To Spot, Avoid, and Report Tech Support Scams

"Tech support scammers may try to trick you with a pop-up window that appears on your computer screen. It might look like an error message from your operating system or antivirus software, and it might use logos from trusted companies or websites. The message in the window warns you about a security issue on your computer and tells you to call a phone number to get help.

If you get this kind of pop-up window on your computer, don’t call the number. Real security warnings and messages will never ask you to call a phone number."

Also see:

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

"Phishing refers to fraudulent attempts to get personal information from you, usually by email. But scammers use any means they can to trick you into sharing information or giving them money, including: 

• Fraudulent emails and other messages that look like they're from legitimate companies, including Apple
• Misleading pop-ups and ads that say your device has a security problem 
• Scam phone calls or voicemails that impersonate Apple Support"

It's a scam.

This fella does not exist and it is a scam, Apple will never send you messages like this most likely due to false notifications in notification center

go to Safari > Settings > Websites > Notifications and make sure to remove all unknown notifications to solve this problem.