I have a very good health food store with a knowledgeable owner. If she suggests a certain root or herb, can I use it safely for high potassium?

Potassium food. She likely knows. See AAKP.com nutrition counter for excellent info on food potassium.
Find the cause. It is important to determine the cause of high potassium and tailor the treatment accordingly. High potassium can cause serious heart problems and it is imperative that you find out why you have high potassium in the first place. Foods are good for adding potassium to the body but not taking it out.
Probably not. High potassium may cause life threatening heart rhythm. Best not to mess around with it. If it is high, standard treatments are: reduce dietary intake, take diuretic such as furosemide, take sodium bicarbonate, take Kayexalate (sodium polystyrene sulfonate) (under doctor's supervision). Important to know why you have high potassium. Kidney failure? See a nephrologist!