Usually I believe two wrongs don't make one right, but at times you have to treat people in the same manner as they behave with you for some time just to bring them back to earth.
She needs to ignore him and take him out of her mind for sometime till he realizes his worth. Sorry for such words but that attitude needs to be dealt with in similar manner. If he were a caring person he would not have left his fiance in a continuous torture without informing her the reason of his anger / depression (if any) that is forcing him to avoid her. He is WELL AWARE how painful the situation is for his fiance that she is confused and wondering all kinds of possible mistakes she might have made, and she might have lost her sleep and appetite because of his behavior.
My request from the sister is to give him the treatment of his own dose. Mind you, he would come running after her within a week or so, when he realizes that he has been given absolutely no importance. That would be the time to know why he was behaving in such an unreasonable manner.