Sorry, but I'm really having trouble with the update. The other girl supposedly slapping the DD wasn't important enough to make it into the initial post? That's a little more than just "drifting apart." If it's true, the OP's daughter sounds like she was pretty rude to her friend, not that it justifies being slapped, but knowing how teen girls tend to exaggerate, I wonder if it was really even a slap or just being silly. Plus, the OP's daughter did accept the apology, so there's that. If the supposed slap was enough to drop her from the trip, it should have happened back when the incident occurred. Canceling on her now is wrong, IMO.
It really sounds like this is one of those threads where the OP doesn't get the advice they wanted and pulls out "extra information" to sway everyone to their side.