Snails are very sensitive to the heavy metal COPPER. My stepdaughter put a penny in her aquarium years ago and it all the snails died in a few days. I later talked to the pet store people that sold us the fish and they told me to never put any copper in the aquarium if I had snails in the water.
You friend may be able to treat that farm pond with some coppersulfate. I use to do this at a lake I worked at every spring. We put the copper sulfate in a burlap bag and dragged it behind the boat all over the lake. The lake would turn green for a few days and any one that swam in the lake got out with Green hair. But it washed out with a good shower.
That may kill any snails that are in the pond. If these worms have part of their life cycle involved with those snails it may help irradicate the worms too. Worms can't survive or reproduce properly if their host (snails) are't around in the pond anymore.
I would encourage you to get a scientist (Biologist or Ecologist) that can identify the genus and specie of those worms. Once you know more about what they are you may be able to get better advice on how to get rid of them.
Oh BTW. The copper sulfate will kill the aquatic plants in the lake also and keep the algae from growing too.
Regards,
Moose1am



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