Sorry to hear that the treatment didn't work, weeds can be very frustrating. It's at least it's good that the follow up spraying will be done for free. I re-read your other thread and the "2,4-D, glyphosate, and diquat" seems to be what is recommended to control water lillies (based upon a quick internet search I did, I'm certainly no expert). Perhaps he didn't get the mix quite right. Keep us posted as to how this follow up spraying goes.
