Most likely they gave her Kayexalate.
Read about it:
http://www.drugs.com/mtm/kayexalate.htmlUsually if your potassium is high the first hour of dialysis starts to bring it down since it is very easily removed from the blood. What often happens is there is lots stored up in the tissue and it starts to transfer over to the blood.
Learn high potassium foods and avoid them: Bananas, Orange Juice, Potatoes etc...