I had a very painful falling out with a really close friend several months ago. Being somewhat older than me, this woman was like a mother and I really looked up to her. When she decided to write me off, I felt terribly hurt and betrayed. I am working on the old wounds this friendship triggered for me in therapy, and am trying to reconcile what was my part in order to not make the same mistakes again.
My struggle is that my boyfriend, who met this ex-friend through me, still wants to be friends with her. We all used to hike and do other activities together and his take is that the breakup was not between him and her, but her and me, and why should he be punished for our mistakes? I find his lack of understanding of the situation and loyalty to me disturbing. Am I off base here?
Given that you feel so hurt and betrayed by your friend, it's legitimate for you to feel uncomfortable if she and your BF maintain a relationship without you. I'm struck by your boyfriend's insensitivity to your feelings.
Although the three of you did things together in the past, the threesome arrangement no longer exists. By virtue of the breakup, everyone has been placed in the position of reassessing loyalties and lines of communication. While your BF may not harbor any animosity towards your once-friend, his priority should be to support you during this difficult time.
Since you are trying to better sort out and understand what happened and why in therapy, it seems reasonable to ask your BF to put his friendship with her on hold. I would encourage you to speak to him further and find out why he is stuck on maintaining this relationship with your once-friend.
Hope this helps.
You may also be interested in reading these prior posts on The Friendship Blog about allegiances after a breakup: "When a friendship breakup spills over to family", "Why breaking up is SO hard to do ", and "Can collateral damage be avoided in a breakup?".