It once happened to me sometime ago, I won't like to share it on social media like cos it was a horrible experience, cos I felted I was betrayed by someone I loved so much and she was the last person I was expecting to put me thru that pain. I quiet understand the kind of trauma u will going thru presently due to ur best friend sleeping with ur girl friend. In my honest opinion, I will advise u to forgive both of them so u also can be free too to move on. Avoiding her calls won't stop her from calling u, cos the damage has been done. Forgive her and take her back if u can!!! But if u can't politely seat her down and tell her u can't continue and keep distance from the guy not as an enemy but as someone u can't confidently call ur friend anymore. He should be protecting u and ur girl friend rather than taking advantage on her cos of ur friendship with him. Secondly, this time u really ned God in ur life, Jesus is the only way to true happiness and ur best friend.