Cutting right to it, my partner has tried to log in after inactivity for a hot minute and he was given a notice saying something about a legacy Password being detected and he needs to update his password.

The problem: the email he originally signed up for is no longer accessible. Does this mean he has lost his account fully and needs a new one? Or are there other steps he can take to recover the account?

I'd suggest filing a ticket and telling staff the situation. Ticket link

he can email subeta support with the relevant info and they'll walk him through the process of proving account ownership and restoring access under a new email address.