Welcome to our forum.
I am sorry to hear about the sorrowful state your girlfriend is in. Perhaps sit down with her and ask her how you can show your support, whether it be showing extra affection, giving her some extra space to work through it on her own, or whatever she needs at this time. I agree that what was done had to be done, and encourage you to not feel too very guilty, but just to have compassion toward your girlfriend, and try to give her whatever support she needs.
If she does break up with her other boyfriend, it will take her awhile to get over it. Even if they don't break up, it will take awhile for this other boyfriend to wrap his mind around the whole polyamory thing. It's not exactly something we were taught to accept and recognize as kids.
I hope Polyamory.com proves to be a helpful resource for you. Check out our various threads, and let us know of any thoughts or questions you may have.
Love means never having to say, "Put down that meat cleaver!"