I think that it's ridiculous that the lent book is unavailable to the original purchaser while it's loaned out. That completely defeats the purpose of an e-book. Edit: Do the downvoters disagree on the benefits of having a book in electronic form? Or do they think that the restrictions are not unreasonable/ridiculous?
This way the book will be available on your device (since it doesn't know it's been lent out) and on your friend's device. Just don't re-enable wireless until you're done with the book or the loan has expired.
This way, at least, I was able to share a book with a friend and we could both read it at the same time to discuss it.