

That system is vulnerable to social engineering attacks. If hackers found out all their favourite things that lead to the core part of the password, guessing the prefix wouldn’t be that hard. Also, what would your friend do if one of these passwords got compromised and had to change it? Would he just add a 1 to the site-specific part of the password?
Yeah, but there are degrees of vulnerability. Otherwise, things like password strength or MFA wouldn’t matter.
If all your passwords are fully random, then that’s one less weakness that can be exploited. People can’t make educated guesses about your passwords just from analysing your social media profiles and history, e.g. if you post a lot about Star Wars, it’s more likely your passwords could contain a Star Wars reference.