People can "study" natural horsemanship methods all they want but sitting on a bucket in a round pen for months waiting for the horse to calm down doesn't seem to be going all that well. Maybe by the time she's 20? Time to put those methods into use, but first you have to get your hands on her, obviously for her own good (that halter) and anyone who has to deal with her.
If nobody can rope her is the round pen made of panels? Can it be made smaller literally forming a chute?You surely don't want anyone hurt including the horse so maybe the owner's vet has some idea for a tranquilizer.
It sounds like this horse would be better off with you for a few weeks at least to get this training process started. Hopefully the owners could then continue with her education at a better pace once she can be safely handled.
Scary situation, actually, for people without a lot of experience. I wouldn't want to tackle it. Good luck and keep us up to date please.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger