I believe she either expected he would eventually leave you and "pick her" (in which case she doesn't understand what non-monogamy is about) or she gave it a good try because she cares about him, but realised it's not working for her.
Either way, if what she wants is a monogamous relationship, and she won't take anything else, it needs to end between her and C, and he needs to be very clear about that, which may be hard if he has strong feelings for her.
I think he needs to tell her in no uncertain terms that she can be with him, and so will you, and that's not going to change, or she can find someone else to be monogamous with her. It might be a good idea for the conversation to be between all 3 of you, if you can through IMs for instance.
A lot of people hope that things will change (that they'll become comfortable with poly; that their partner will become mono) but it an be unhealthy to spend too much time hoping for things to change when they don't. I think it's good that she realised it wasn't working for her, but I don't think she dealt with the situation very well.
You might want to try resuming communication with her, asking her if she wants to talk with you, if she needs help with anything. You might be scary for her, as you're the one who is engaged to him, and so if you hold out your hand, maybe she will try to reach out as well.
Good luck, but I have to say in my opinion it's likely the relationship with her is going to end. She would need to be comfortable with polyamory, and she certainly doesn't seem to be.