I may be wrong, I didn’t test it, but I think that if you create different wix accounts with different emails, from there, you’ll have the possibility to do so.
As for having different admins including himself for each, that’s a simple “shot” You just need to add “The User” as admin to the individual wix account that you’ll have created for this sub-domain. Your customer stays owner of the accounts and sites, therefore he controls everything.
So let’s say, for example, that he has MyWebSite.com as an owned domain.
That one can be associated to the main wix website that, I suppose, is already created.
From there, you can add SubDomain-1.MyWebSite.com by creating the subdomain in your owned domain manager, redirecting it to the 2nd account you’d have created on wix…
Repeat with SubDomain-2 redirected to 3rd account, and so on for EACH sub-domain.
One important point though, is that, if I’m not mistaken, you’ll need to go with at least the basic upgrade to be able to use the wixsite.com domain on EACH wix site. Which means that if he wants to have 8 sub-domains, you’ll need 9 upgraded accounts, 1 for the main domain and the 8 sub-domains.
All depends on the customer’s wish to pay for upgrades or not, but it’s feasable
Anybody want to add to my theory?