There are three types of plants that can be easily mistaken for common water lily. You should become familiar each type before you chemically treat the "water lilies" in the 1/4 acre pond. 1. fragrent white water lily, 2. cow lily or spadderdock, smaller roundish yellow flower with big green leaves some may be held above water, 3. water shield- no slit in roundish floating leaf and stem attached in middle of leaf, small maroon to purple flowers.

Rodeo a glysophate product is the best herbicide to use for lily control. A surfactant should be added to a pure glysophate. I am not sure if this year there is a glysophate product available with premixed surfactant for pond use and packaged in smaller quanity (pint, qt). Sepro Chemical company makes an AquaPro in 1 quarts but I think it may require addition of a surfactant.

Kelly Duffie may be able to give advise on this topic. First determine which type of lily your friend has in the pond and then reask the question for best herbicide. I will do a little research in the meantime.

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management