Having different credentials would actually be faster. One key chord to ask 1Password to fill in my password and log in, and I'd be done. Instead, with SSO, every time I need to log in, I need to type in my email address, go through a bazillion redirects to Google where it asks me which of my accounts I want to use (even though I already typed in my email, and even though I only ever use one of those accounts for SSO), click it, often have to then enter my Google password and/or OTP code, then go through a bazillion more redirects back to the app.

There are also lots big companies that have "SSO" but it is half-baked or incomplete with regards to which services and you end up with MSSO - multiple single sign on.

Install 1Password and it's basically like having SSO.

And why shouldn't he have different credentials? A password manager would make this a cakewalk.