Share/vent with other like-minded members? I'd do a "public" announcement that so-and-so has a better party to go to (said with heavy sarcasm) and needs someone to help her with her obligations by taking over the secret Santa gift buying for Christmas Eve and ask for volunteers to do that and any other things she was supposed to. I wouldn't ostracize her outright but I'd sure make it warm in there for her when she "comes back".