Perhaps the leader of your group can either speak to the entire group before leaving for the retreat, or if nothing is scheduled send out an email or letter to members stating that the retreat is someplace that quilters will have less or no one there to help with issues... something worded like that. Or... simply take this individual aside and talk with them one on one. I have had this happen in a classes I was teaching machine embroidery. It is a very difficult issue to deal with. Especially, when the person throws an actual temper tantrum in class.... very embarrassing for all! Luckily, she left after that!! I hope this situation can be resolved for you so that everyone involved in this retreat can have a nice peaceful time.. or should I say, pieceful..