Assigning a domain to a subdomain?

Hi! Sorry if in wrong place, first time on forum.

So I’ve recently set up my raspberry pi to host a www.ads-software.com network.
I have one main site and one subdomain to that site.

I’ve used the main site for myself and designed a site for my friend using the subdomain. Is there anyway i can now put my friend’s already purchased domain on the sub domain site so the site, while still on the same raspberry pi can be used via her domain.

Sorry if that’s a bit confusing.

It this isn’t possible, is there a way that i can host multiple separate wordpress sites off one raspberry pi?

Thanks!!