Again. Read. The. Message.
If it was because it had expired, he would get an "Activation server not available" message.

. What's so hard about that point?!? I'm not going to argue with you - because I know I'm right. Just read the message again. The expiration date is just a trigger, here's what happens:
- iOS 7 has default deactivation date.
- iOS 7 periodically checks activation, and also checks it on the deactivation date
- When this date it reached - the phone must be reactivated
- The server will reply with (essentially) one of 3 things - "activate", "expired" or "invalid uuid".
Case 1: Activate - the phone is reactivated, and the same check is performed every 24 hours.
Case 2: Expire - the beta has expired and hasn't been renewed, and thus the activation profile is revoked.
Case 3: Invalid UUID. If a reactivation request is sent from a device which is unregistered, it is flagged against the activation servers as being invalid/OOP (out of program). An OOP device cannot therefore be reactivated using the development servers until it is re-enrolled in the program.
Before now, Apple have never really enforced Case 3 for iOS 6 or 7. They are now, however. Which means that when a beta expired, or your phone does a random activation spot-check - its activation profile will be revoked and marked as OOP.
And no, apple aren't devoting their manpower - but a server somewhere obviously is

.
Also - how exactly has he bought it - the developer centre is still down!