Thanks for everyones help with this. I got him to remove it as we had just put our house on the market and I from what you have said, this would cause all kinds of issues.
There used to be a drain away near there apparently but it's under where he has extended out his house to enclose the alcove, hence why he wanted another one. Makes sense and I think he'll look into it a bit more now before trying again.
Thank you once again as your comments made it a lot easier for me to tackle with him.