I think like other people have said, you did end it and he has to get on with his life, you cannot expect him to stop being mates with these people, but i think your supposed 'best friend' is betraying the rules of friendship! I think you need to be onest with her, its the only way you will feel better! You need to get it off your chest, even though you are talking and getting advice from people off here, we cant do anything to help the situation. Your in the driving seat, you have the control! I was in a similar position and i just let them get on with it, and it has ended in us not being friends anymore, that was her choice, after time i was prepared to over ride it, but she couldnt face me after they ended! She knew she had done wrong, but it took her until they ended for her to see that! Now me and this lad talk alot and are quite good mates, and she hates it that me and him talk, but i new him long before her, she needs to stop being so petty!
Good Luck, hope it all goes well!
Just to what you think is right, thats all you can do!