She's wrong if your pH is 8. That's fine. And as
@Mmathis said - more important is that your pH stay stable.
I really didn't do a great job of explaining the filtering concept did I? In your situation the best set up would be a plastic tub or trash can with holes drilled near the bottom - one side only. Fill it with batting and set it OUTSIDE the pond. (I guess you could put it in the pond, if you had somewhere to set it where the drain holes would be above the water level in the pond. Pump water from the pond up to the container and let it drain down through the batting back into the pond through your drain holes. A laundry basket would work best if you could put it under your waterfall and just let the water flow through with most of the basket out of the water.