OP, I think it's great you were able to take DD to this anyway, so she can still have some fun at the event. I would be extremely disappointed with G and I don't think I would want anything to do with her anymore. Not necessarily a cut direct as in, if you saw her on the street you'd walk on by ignoring her, but no need to help/encourage DD to be her friend any longer.
As you say, you've been accommodating to her in the past, and she hasn't reciprocated much, and now she's flaked out on something huge at the last minute. I'm not saying her religious beliefs couldn't be genuine and sudden, but if they are, she had a lot of other options besides doing what she did, such as going with DD to the hotel and socializing with her and the other girls but not actually attending the convention, or offering heartfelt apologies to DD for her disappointment and inconvenience.
I think you were okay to talk to the girl directly, for clarification of the sudden change in plans, since it seems like they were going to have a very definite effect on you--either you were going to be driving DD to the convention (which you did) or you weren't going to have the house to yourself that weekend any longer. I would want to get clarification personally too in that situation. I get the grey area with relations between young adult children; but the other girl is a legal adult, going to college, so she should be able to handle talking to another adult and hearing how her actions have inconvenienced her, not just her friend.