At this point, you have taken every reasonable step to return the dress to its owner. If she does not respond within the 30 days, you should be able to dispose of it any way you please.
If anyone from the "Worse Crime Ever" crowd gives you a hard time about disposing of the dress after the 30 days, remind them that the former friend cut you off. Tell them that your options are limited because she wants no contact from you. You are concerned that she might feel that you are harassing her, if you send her any additional correspondence after the thirty-day period. Besides, you really don't want to have reminders of her after she cut you off. Tell them the dress reminds you of her and it is better for you emotionally if you no longer have the dress.