Llechwedd, I popped down to have a look at the stream that you are talking about and it must come from Cwm Mountain somewhere. It actually flows eastward and is exactly as Dave has said. The stream goes into Ffynnon Fair and continues along the edge of the copse before it was cultivated under the road next to a house named Glan Aber. It then goes into the field and into a wooded area that has a pond from there it will go under the A470 at some point.
I went on to Google to have a look at the stream from the air and it is clearly visible once you get your bearings.
I don't know the name of the stream but there are a number of people on the forum from the Deganwy historical Society who know a lot about the area and may be able to help.
Now your question has raised another for me as I undertood that the Afon Wydden flowed from Glanwydden to Bodysgallen Hall and continued past Marl and came out by the Rail Station in Llandudno Junction. However I stopped in Glanwydden today and had a look at the Afon Wydden and that was flowing eastward
in the direction of Afon Ganol so I'm more confused than ever.
The only thing that I am certain of is that your friend will have no help whatsoever from Mostyn Estates, my thoughts on them are unprintable! :rage:
Water courses though can be the responsibility of Dwr Cymru so your friend needs legal advise.