The bottom line is...things happen!
Buy another tube of banamine, be happy your horse is OK and move on from it.
No matter how mad you get, none of it is going to make it go away.
Obviously the BO was not clear to the other boarder if the BO says she told the other boarder not to bring your horse in. Or the other boarder is just a bad egg and this is still the BO's problem because that boarder is still there and the BO is allowing them to do things to other people's horses.
I would think that a change in routine would be a bad thing for a horse that is prone to colic issues, honestly. So leaving him out when he is used to coming in might not have been the best either.