My boyfriend is still upset about an ex??

So, my boyfriend and I have been together for a year now. A couple months before we started dating, a girl that he was talking to disappeared out of his life. She cheated on him multiple times, dropped out of school, and just disappeared. He found that out all in one day. He doesn't speak a lot about her, only when he is making a serious half-joke about being hurt or being cheated on. It's not to the point where it's unhealthy or obsessive and I should be worried, but it's to the point where it's been so long since it happened that I don't really know what to do, I don't know how to help him. Recently, he told me that the only reason why he can't get over it is because he didn't have proper closure. He saw her Facebook recently and realized that she blocked him. She's still with the guy that she cheated on him for. He said he wishes that he could tell her how horrible of a person she was and how much pain she left him in, and he said he almost wishes to warn her boyfriend too.