With Sony, if your books are DRM free, you can just drag them onto you Reader (or the SD card), using no software at all (as if it were a USB stick). You can also use this free program:
http://calibre-ebook.com/
to manage your books, switch formats, and send them to your reader. Sony also has their proprietary software, which you would have to use if you bought books from Sony.
After reading through the posts, I think that the Sony reader might be a good fit for your dad, but I just thought I'd mention (since you mentioned that he lives in South America) that Barnes and Noble now carries e-books in Spanish, if you are interested.