I think it depends on your client’s hosting provider. If you use shared hosting with the domain name included, you sometimes cannot point the main domain to anything else than the web root. Then in such a case, you have to live with that and certainly not move anything outside the web root anyway or persuade your client to move to another provider, or keep domains and hosting provider separate (that’s actually what I do with my personal domains, they live separately from where I host my stuff, so I can point them to whatever server I want).