App store id different from OSX system id?
You're talking about the password for a specific Apple ID (which you use on the App Store), versus the password for a specific user account on the Mac. There's no requirement anywhere that they have to be the same.

Your dad probably used a different password for his user account. If you cannot remember it, you can reset it by following the instructions here:

https://support.apple.com/en-us/HT202860