But even with friends you wouldn't just break up because of a relationship unless the relationship was very unhealthy.
I sincerely doubt your boyfriend will end things with her. Even if he does,her presence will linger in your relationship for a long time.
All the agreements in the world will not prevent chemistry and passion fueling feelings of love and intimacy. You can call it FWB but they will still have a connection that will be unique to yours.
I agree on some things, ofcourse you have to judge on part of the information, since it is almost impossible to get all the info.
I do know that my boyfriend doesn't want to be in a situation where he is hurting two people. He is hoping to be able to continue it as a friendship. However he doesn't want red in a position where she is still hoping for more. And indeed we have a lot of talking to do to figure things out.
My boyfriend doesn't want to continue a fwb with red in this situation because then we will keep the situation the same. My boyfriend never told red that if things would work out between us that he would want a relation with red. That is something he also told me for a long time, that he feels it was more a friendship
In the end all 3 of us made mistakes and we all have to deal with the crap that come from it.
I honoustly believe that we can work things out.