I feel for you both. I wish she would have talked to you the whole way through the process of meeting the new guy. You may have been able to help her introduce the topic and transition. Since she threw you and poly under the bus, now you have some healing to do and some trust to regain.
I feel for her because the longer I am poly, the less I anticipate people's negative reactions. But they still happen. I'm only seeing 1 guy, but he has other girlfriends. I usually call it non-monogamy, then follow it with "everyone has full knowledge!" Still, among the few people I have now told, their reactions weren't, "Congratulations, that's great!" I haven't gotten a single one of those. LOL. The misconceptions and leaps are mind boggling.
I told a friend who is usually interested in my love life. She no longer asks me about who I'm seeing. My best friend from highschool who is a doctor immediately asked if I was practicing safe sex and getting tested. Not, "is he cute?" The first and only guy I have had to make aware of my relationship status thought non-monogamy meant I was promiscuous - and tried to take advantage of me as a casual sex partner when I specifically said that was not was I was looking for. I later found out he also assumed that meant I regularly had group sex. Huh? I talked to him for hours. How he got there, I don't know.
I hope everyone is communicating better in your group. Sometimes the tough learning experiences pay off more dividends in the long run. Best of luck to all.