ETA: Thanks for clarifying, OP!
I was thinking that DD's sharing of the room was always contingent on G being there, too, and that--conceptually, at least--DD really didn't have an agreement with the other girls, only with G. Of course it depends on what DD and G agreed, and on what G and the other girls agreed, and we may have no way of knowing those things exactly; except that as the OP said, the other girls all blame G for messing things up.
I wonder how the situation would look if G had had to drop out at the last minute because, say, she broke her leg? Her sudden, voluntary flakiness and lack of respect for everyone else's plans cast a big shadow over the rest of the situation... but fundamentally, is it any different than if a medical emergency had forced her out? Just a hypothetical.