As far as advice goes, I think the best one I can give you is a friendly reminder that all things work out in time and those that are meant to be will be, regardless of how much it's being pushed and pulled.
What I can tell you, first hand, is what it feels like for me (in the place of her) in my relationship with my husband and his girlfriend. Initially, I took it horribly. I thought everything from screaming to leaving. Then I became very angry, and I wouldn't talk, at all, to either of them. I felt betrayed, even though I knew deep down nothing was going to end, everything will be fine, etc. I still felt betrayed. There wasn't enough warning, there wasn't enough tact, there wasn't enough time before one thing led to another and the list goes on and on.
Now, I look back at my reaction and while I understand fully why I behaved the way I did, I also realized that nothing could have been done to make it "right" or "easier" with me. It's just my reaction and my coping method. I had to let the fear and anger out before I could clearly and objectively see what I wanted.
It's a work in progress, I find myself repeating. It's going to be alright, you all just have to be very open minded, tolerating, patient and communicate!