Something like the image attached.
Well, I know there is a limit to how many ores can be in a single vein - so that might debunk this theory. Also, like you stated, the chances that such an event actually occurring would be basically 0, so it doesn't really matter in the long run. I'd be interested to hear from some other people who might know more about this than me.
This is impossible unless you have the login details to your friend's account. If you do have his/her username and password, you can open Minecraft twice on the same computer - with one logged in as the host account (your account), and one as your friend's account. After you do that, you can do as you please with their items, etc.