People make mistakes and that includes people at AA meetings. To expect that everyone will be perfect in adherence to the rules is silly. We cannot control other people---only ourselves. I suspect that this bothers you because you were complicit. You did not tell him that you did not want to hear him break anonymity. If you had spoken up, I am sure this would not have weight any more.
That said, there is a lot of confusion about what anonymity is. My sponsors felt that we needed to remain anonymous in the program and not share identifying information with anyone I met in a meeting. Others have said that this is okay. People interpret this differently and he may not have felt he was breaking anonymity if he was sharing information with another alcoholic at a meeting.