Boyfriend's childhood friend

Last year I caught my boyfriend having inappropriate chats with a childhood friend of his online. They were saying that they wanted and loved one another and could not wait till they saw each other again. I confronted him about it and he states that he did it on purpose because he knew that I was suspicious and snooping. In essence, he said he wanted to teach me a lesson. I almost broke up with him over this but he was completely devastated and swore that he had no feelings for this girl, he was sorry that he hurt me, he only did it to punish me and he knows that he was wrong. I have tried to put it behind me but a year later I am disturbed by the fact that he and the girl are still friends. She lives in another country and he swears that it is platonic but it burns me up and makes me angry that he is still talking to her. I have already spoken to him about it and told him that although I can’t tell him who to be friends with he should try to put himself in my shoes and then do what he thinks is right. I guess he has decided that remaining friends with her is okay. He argues that they have been good friends since childhood and he would feel bad hurting her feelings. To make matters worse they are both returning to their place of birth in a few months for a big reunion. The fact that they are going to be seeing each other face to face is truly messing with my mind. Should I let this go? What should I do? I do not want to lose our relationship and regret it later on the other hand I feel like I should not allow myself to be disrespected.