I've seen apps that can read FB2 books on the iPad, so I am not too worried about it.
The reason I specified that this is for my dad, is because I want something that's simple to use. Personally, I would never buy an iPad. I don't like how closed Apple's platform is.
A while ago, when I was buying an MP3 player for my mom, i chose an iPod over a much better iRiver player because of ease of use. I am using the same logic now.