Kobo Desktop seems to be a very closed environment: I can't see any way to use it to open .epub book already on your PC.
I would have expected that the sync button (top right) would download all your Kobo books into Kobo reader without the rigmarole of going via ADE, but I can't verify that.
If your dad gets used to ADE, he will also be able to use it for other things, such as borrowing and reading library books.
And don't forget all the free (mostly older) books you can get from sources such as Gutenberg http://www.gutenberg.org/
(he can read those on ADE, or Calibre, or most PC epub reading software except Kobo desktop).