I don’t have the opportunity to listen to that audio link…
But the difference between Dr. Ray (as I presume he is saying in the file) and this situation is that Dr. Ray has these rules laid out and established and understood by his kids AHEAD of time.
In this situation, the OP’s son has a “private journal”, and the fact that the OP questioned her judgment of reading it means that she DID NOT establish this rule ahead of time. Her son is under the assumption that he has privacy since this was not discussed and understood ahead of time.
SHOULD there be a rule of “no privacy”?.. Maybe! But that’s up to each individual family to establish those types of rules… *ahead *of time. Unless rules are established, discussed, and UNDERSTOOD, they aren’t really “rules”, now are they?

If they aren’t established and understood then the son’s understanding of his “rights” have indeed been violated. From his point of view, he probably will trust his mom less in the future.
Again… maybe those rules SHOULD be established, but considering the entire premise of the OP… it sounds as if they weren’t. Since that is the case here… she probably owes him an apology… and then SHOULD establish rules for the future.