Even then, the Kindle doesn't read ePub (the last I looked), and the iBooks does not read azw or mobi. The only format that both the Kindle (device) and iBooks have in common is PDF: There are no PDF books in the iBook Store, and few to none in the Kindle store.
On the other hand, you can read just about any Kindle book on the iPad using the Kindle app. Just sign on with the wife's account in the app. No way to do things the other way around, DRM or not. Unless there have been some major updates to the Kindle devices.
I wonder if the Kindle Fire has or will have an ePub reading app?
Addendum: There are other book reading apps that will read the .mobi format. I should have mentioned that; but since the Kindle app reads Kindle books, it shouldn't matter in this case.