So it looks like the second scenario in my post.
Since he had been using the old client, it might have created a "non-standard" transaction and that Tx was rejected by his peers,
He's the only one who can see the transaction (or users with old client), that's why it's not visible in blockexplorers.
I understand what you explained, it's really good. But is he not the only one who can see the transaction? As soon as he uses the new client, his transaction is still there, he can manage to cancel it. The transaction is still there but not done, right? I mean people who can see transaction? Only him?