Hi, bad breath in dogs is usually caused by bad teeth...tartar, rotten teeth or gum disease, but can be caused by more serious complaints such as liver or kidney failure/disease. Either way it would be best to get her seen by a vet. There are lots of ways to coax your dog into taking medicine; crushing into food, hiding in food or placing the food in her mouth and rubbing her throat to encourage her to swallow [only do the last one if you are sure of the dogs temprement], but get her to the vets first and see what they advise.
Good luck
Lisa x