They are Muscovy ducks. They’re the quietest of the ducks and are usually raised for meat. I doubt your neighbor intends to let them out if s/he has no perimeter fence. Also the choice of the quiet Muscovy breed suggests s/he doesn’t want to trouble neighbors with unwanted noise.
Ducks in general (not sure about Muscovy ducks as they’re different from the usual domestic breeds) are mainly only a danger to young succulent plants and low fruits. They’re very fond of garden pests, so later in the season they can be a lot of help in the garden. They won’t damage lawns unless there are too many ducks for the available space.
Go and talk with your neighbor. I’m always about assuming the best... most people you meet really do want to get along. You can get the best answers to your questions and concerns that way... and who knows? You may meet a wonderful new friend.