Hey Ken. Many options exist for managing duckweed, but as Eric mentioned, duckweed's aggressively prolific habits often make it a challenging adversary.
From a herbicide-approach standpoint, find out if the pond encounters frequent or constant water flow-through. If it does, it probably is NOT a good candidate for one of the most consistent herbicide treatments; that being one of the fluridone herbicide formulations. A possible alternative is one of the aquatic labeled flumioxazin herbicides, since they accomplish their function with a significantly shorter contact-time; but completely lack the longevity of activity that fluridone affords.
Most importantly!!! Be aware that NY State has some of the most restrictive herbicide regulations in the US. I'm not even sure if either of the referenced chemistries are labeled (legal) in NY; so that would definitely be something for your friend to investigate.
If herbicides aren't permissible, he may have to resort to mechanical/manual removal - which will require frequent repetition, quite possibly every season since it will be virtually impossible to break the seed-production cycle through harvesting means.
Good luck!

Kelly Duffie
Cypress, TX