Re: I really don't see...
As I understand it, it's because iMessage doesn't send SMS to a number, once it knows that number is an iDevice, it uses a proprietary Apple format.
Apple isn't intercepting SMS messages, it's just not forwarding on their proprietary ones and her friends aren't sending SMS. Imagine it a bit like you using Skype, then you stop using Skype but all your Skype friends still sending messages to you on it. The main difference is that Skype makes the difference between SMS and Skype messages very obvious whereas iMessage hides it from the users and "auto-detects" if the receiving number is an iDevice, but needs it to be manually deregistered by the owner of the receiving iDevice if the number is used by a different phone vendor.