I sense that you feel very violated by SO's breach in your R's security. Today I attended a professional seminar about ethics and boundaries when working with clients. There are four types of boundaries: Emotional, Psychological, Physical, and Sexual. In today's society, the modern version of a diary is blog. The problem with blogs is that they are out in the public domain and anyone who is fairly savvy with a keyboard can access them at anytime.
Martha...are you able to pinpoint why you were upset by SO's admission? The lie is one part...but what was the actual trigger? Did you feel invaded, lack of respect, or not having your boundaries respected as an important part of SO's life? Did you two ever discuss expectations in this new R together? Have they been verbalized and understood by the two of you together? Are you two on the same page when it comes to expectations and boundaries?