You said yourself that she does have mites, they literally drain the life out of them! You don't have to try everything, but try something. Livestock dust is $5 a 4lb bag, isn't it worth a shot to save your lady? Just my opinion, they are your birds and you can do what you want.
DE isn't going to do anything to get rid of the mites. Get some livestock powder or sevendust (something with permethrin in it) and give them a good dusting. You're going to have to do the coop to after you get rid of all the bedding