Whilst cranberry is good for cystitis in humans, I wouldnt know if its so for dogs but Im sure someone will

, if you have an attack you need the doctor. No question, because no amount of gallons of water or berries will help when you have this infection. Well, it helps lol, but you need antibiotics to clear the infection. So, I would think the same for a dog. What worries me is that she needs it to be confirmed that it is cystitis, in case its not, and also in any case she needs treatment from the vet for it asap. Its painful to say the least, and can rise up to the kidneys and cause all sorts of probs.
Let us know how she is, and best of luck