Prompting for Download

I helped my brother install WordPress for a blog. Everything looks great to me, but he’s having some people telling him that when they try to view the page, it prompts them to download the page/site. (It says the directory name where WP is installed.)

Any ideas what might be wrong? The only thing I can think of is that it’s an older browser with a setting wrong, but I don’t know what to tell him.