How does she sleep at night?

I have been friends with this girl for 20 years, more closely the last 12 years, and we were together for about 3. We have an excellent level of comfort with each other, we know each other inside and out, and there is never an awkward silence between us or never not anything to talk about. We just have a fantastic relationship on so many levels. We recently split up over some trust betrayal on her part. Something I never thought she would do, but nonetheless. She recently became close friends with this guy that she betrayed my trust with. I told her that I did not want to lose her as a friend even if we couldn't be in a relationship anymore, but I am making her choose between having me as a friend (who has been there all these years), or him who she has known for the about 2 months. I don't think this is too much to ask, do you? Who would you choose?