Installing in subdirectory vs. subdomain?

I’m working with a client who knows very, very little about web development/design. He has decided to stick with his old, bad webhost instead of one of my recommendations, so this has been difficult and it’s not even installed yet! He doesn’t understand much at all when we try to talk, although he designed what’s in use as the current site.

I had all kinds of problems even accessing his webhost account. It doesn’t have any kind of modern interface: no cPanel or Plesk.

I asked him to set up a subdomain when I couldn’t access that part of his account. Instead he set up a subdirectory.

His current site is a Dreamweaver build. Does anyone know if there will be any problems with making this new WP subdirectory into the main directory when I’m done? I’ve always done a clean install before and don’t know if I should make him back it up and try again.