Poly relationships are no different than any other--they form, prosper, and may dissolve. Your girlfriend is correct in that her relationship with her girlfriend may have dissolved had you not appeared.
The only things I'd suggest are these:
First, get them together to talk about it. Yeah, it's that open communication thing. It still applies when it appears a relationship is dissolving. Indeed, it's as critical in the last stages of a relationship as in any other part. Negotiating a breakup is like negotiating any other part of a relationship.
Second, you're dealing with two people who will need support. Unless one of them has done something dastardly, it's best not to take sides. One is your lover who will be dealing with some grief. The other is a friend who will be dealing with some grief. Find a way to support each of them and steadfastly refuse to take sides in the matter.