Brad you were just blessed with a plant that is difficult to control when it comes into an environment that helps it thrive. I know every herbicide on the market and know that most treatments will have minimal results. Have him hit it with Hydrothol. Fish kills may happen but you can also restock later in the year. Many of our customers in Michigan use a product called Nautique for Wild Celery and all it does is knock it down out of the way for a month or so. Most of those treatments are after the plant flowers in July. I think I know your applicator if he is local to you.