I dunno, given he was sorta careless with is supposedly very personal and private writings, it could have very well been left there intentionally. Going through AA steps doesn't make anyone immune to whatever fall out from what they have done while drinking no matter how it is discovered, whatever step they are on.
What is the bigger hurt? The fact that, gosh you read his diary/journal or that he did some risky and possibly damaging things on an ongoing basis? What if she had read something about how he victimized a child or something?(going to extremes here but you get my point). Is it still protective information that the partner should overlook? Hardly.Consequences will still happen and that is why the disease is so icky, even when healing, there will be residual pain.
Plus we are all assuming that her reading this is a BAD thing. Painful, yes but sometimes when this kind of stuff comes to light in a marriage, it can actually be healing and begin conversations that wouldn't have happened otherwise.