First, you need to be assertive and let your friend know that although she means well, the horse is your responsibility and you are the one responsible for her well being. Let your friend know you are grateful for the board, but you need to have control of what the horse consumes.
As for what your horse needs, well, if you are feeding high quality hay, she could be getting what she needs, but you can always add something like Smartpak or a multivitamin if you think the hay may be deficient in nutrients. I have my easy keeper on 2 flakes during the day, a Smartpak Easy Keeper vitamin, and a few hours grazing with a grazing muzzle. An easy keeper won't need beet pulp, chaff, grain, oil, or anything else!