I'm going to tell you what I'd do, having been in the new girlfriend situation in a triad, and also having been in the "old" girlfriend situation, myself...
Here's the thing. The other girl probably is really nervous about upsetting you (after all, that might earn her negative points with the guy). Also, she clearly wants to be accommodating, and probably has a lot of thoughts about how if she really loves him, this is okay, and how things will just get better... What you said about the jealousy from him really strikes a chord with me. I have been in exactly that situation, where my guy doesn't want other people to think he's jealous, but is actually totally jealous. It sounds to me like this other girl is really confused about her own wants and needs, and has been conflating his wants/needs with her own. It's really easy, when you love someone and they're very assertive, to confuse their wants with yours. Since poly is all about throwing out the "rules" of relationships, it's also easy to get into situations that aren't exactly healthy, without realizing it.
I really don't recommend taking the things she said to the guy you're both dating. He might end up upset with her (even if only mildly) about things, and I think keeping her trust is more important. I think the best thing you can do at this point is be really supportive and open about your feelings, and encourage her to share hers. I suspect that she still worries about trusting you, especially if she actually is getting the short end of the stick where poly is concerned. It really sounds like she would prefer for some things to change, and until you know what those things are, it's hard to decide where to go from here.
In summary: I think the best thing to do now is to pursue information gently and discreetly, and speak and act with caution.