Just got a (free) Kobo book using a web browser.
When I launched Kobo Desktop, it automatically downloaded it to C:\Users\<username>\AppData\Local\Kobo\Kobo Desktop Edition\kepub (under a silly name),
and I could read it on Kobo desktop.
When I renamed it to 'test.epub', I was able to read it on a regular reader; it did not appear to have any DRM associated. Indeed, it wasn't a Kobo edition at all, but a Gutenberg one. However, I've just experimented copying (nonDRM) .epub files into that same directory, and Kobo Desktop doesn't see them.
So, back to your original issues. It looks as if your Kobo books should just automatically sync to your Dad's computer when he opens Kobo Desktop, and he should be able to read them on his Kobo Desktop.
He won't be able to read any books you aquire by other means on the Kobo desktop.