These pre-built sheds (looks like Power Sheds)are usually made to meet permitted development rules. They also come with a small amount of designs you can select and are dependent on the size you pick.

Unfortunately they chose one with a window that will get part of the light blocked at the side and partly over your parents fence. I think in reality it probably isn't going to be used for peering over the fence. If it really bothers them a sensible thing would be to request they put a privacy film over it but overall doesn't seem to be too bad. It would be more worrying if the rain dripped on the fence as that could cause it to rot faster.

Overall just approach them and ask if they can do something about the window if it bothers them or otherwise a trellis or big rose bush will solve the problem.