So, the receiver's bitcoin-qt wallet syncing the 'forked blockchain' through the sender's wifi? I'm really not sure, but I don't think that's really possible (apparently, it is). But if it is indeed possible, I assume that it would be very difficult to pull off, and the sender probably needs to do some stuff to the receiver's wallet for this to work. But again, I'm not sure.

Very interesting question nonetheless. Let's wait for other replies.