Cupcakes are a kind of art form as well.
Not associating with her doesn't make them her bitch. Many couples step back from group activities when they become more serious. Putting on a brave face and spending time with someone you don't trust seems more bitch-like to me. Either way the only person really suffering will be the poster. Just depends on which is worse. To me it would be worse to be around the chick and be the only one that knew how she really was.
Not associating with her is different from not associating with all of her own friends. If the attempts to take things doesn't work, the manipulator will eventually move on. If the one manipulating notices her target is becoming increasingly withdrawn, all it does is give her a reason to keep pressing on.
"Putting on a brave face"? Is it really that hard for you to be around people you view as untrustworthy? With enough time and persistence people will see who that person really is, even if the group may need a nudge every now and then to notice it.
I see no reason to run from someone like that. Their power comes from how you respond.
It's not the untrustworthy part that's hard for me to be around, I don't trust plenty of people, it's that someone is seeking to undermine and destroy a relationship in a subtle fashion from the inside out. A sociopath wouldn't be obvious in their goal and as someone before me said, the group would think she was paranoid. I don't think we see this in the same way. To me, it doesn't seem like running, more like avoiding drama and allowing the group to see this person for what they are.