Mink, raccoon, otter, heron, stray cats (domestic type and wild type like linx or bobcat), fox, black bear.
I suggest your friend place a camera or two around their pond. There are many types available these days. I have used a trail camera in the past, but now I have wireless cameras that I can view on my phone. These are much easier than the trail cam because they record on the cloud. The trail cam records on an sd card that you need to pull and plug into a computer, so it's more difficult.
Of course, no camera will prevent this from happening. The best thing might be to cover the pond with netting or some type of greenhouse structure.
