What to do if my boyfriend's ex is trying to get him back?

My boyfriend left his facebook open on my laptop and I saw that his ex-girlfriend sent him a message. I know I am a horrible person for reading it but it was driving me crazy. To sum it up, she was throwing herself at him. She apologized for treating him poorly and claimed to have an eye-opening, life-changing experience when he broke up with her. She said she loved him more than anything and asked him to consider changing his mind about being with me and going back with her. I never should have seen it but I'm irritated nonetheless. I trust my man and know that he won't leave me, in fact he asked me today when I want to get married (before or after he finishes law school), but I can't stand the idea of another girl all over him. Should I just let it go or should I tell him that I saw it and that it bothered me? I feel horrible for reading it... But he left this girl for me and it would absolutely kill me if he ever went back to her. If I believed in soulmates, I would say that he is mine.