My boyfriends friend (20M) won't stay out of my (20F) personal buisness and I'm not sure what too do about it

6 months ago, a mutual friend (I'll call him M) Was messaged by a girl claiming that my boyfriend had been in an online relationship with her and had suddenly blocked her. M came and Told me and obviously this ended with me discovering I had been cheated on.

M is my boyfriends best friend. The first thing M did was tell the rest of our friendship group and his family before telling me. He also befriended the girl my boyfriend cheated with and has frequently had a go at my boyfriend for hurting her, I rarely get mentioned by him.

It has been 6 months, I put a lot of effort into forgiving my boyfriend & he worked hard to earn my trust back and we have come out of the other side of the situation. M is still hung up about it, at least once every month since it happened he has messaged my boyfriend saying he doesn't want to be friends with him anymore because he can't work out how to forgive him for what he did, but it never lasts. The rest of the friendship group never brings up the situation, and we have seen them countless times since it all went down and everything between them and my boyfriend is normal.

Obviously he's allowed to be angry at my boyfriend for what he did but in my eyes he has almost inserted himself into a deeply personal situation. I confronted him recently, asking if he could apolagise for telling the rest of the friendship group + people I didn't know before telling me because it was humiliating for me. He said it became a group issue as soon as he found out (and another friend who was also messaged by the girl) and it was basically the straw that broke the camels back. It is also tearing apart the friendship group as now my boyfriend and M basically have shared custody of the rest of the friendship group as M refuses to hang out if my boyfriend is there, so it's extremely difficult to make plans.

I was the one cheated on, he does not have any place to 'forgive' my boyfriend because he was not the one wronged. He went out of his way to share something extremely personal with people I didn't even know, and has now consistently used what happened between ME AND MY BOYFRIEND as ammo in arguments with him. But at the same time I don't know if I'm being a bit blind and maybe he does have a right to forgive my boyfriend? Idk, I just want him to stop bringing it up all the time.

M flies off the handle quickly and I'm a bit scared too confront him, but I desperately want him to just let it go or at least stop bringing it up. How can I approach the situation?