Take a lemon and slice it as thin as you can get it. Put the slices into 2 cups of boiling water, cover the pot, turn the stove off and let it sit overnight. The next morning strain out the lemons and pour the water into a spray bottle. Spray her twice a week and before going outside for long periods of time.
If she is already itchy from flea bites boil some dried rosemary leaves and spray that onto the bites. Rosemary is a natural anti-inflammatory that will stop the itching and scratching.