It could be fungal mycelium, which might be the cause of the problem but more likely not. The grass dying could be due to the conifer taking all the available water.

He could have lawn grubs. I doubt the conifer has anything to do with it. It wouldn't be just a patch. Too much fertilizer will burn the grass and cause brown spots too. That might be the white powder you are seeing.

Grubs (or some other critter) eat the roots. This will certainly cause a patch here and there to turn brown. I use a product called "Captain Jack's Deadbug Brew®." See if you can find something like that.

The layer of white may be unrelated to the death of the grass - there are many causes for brown, dead patches, but all have accompanying symptoms which point to which problem it might be, some of which may go unnoticed until the grass dies. Did he notice any other symptoms prior to the area dying back - like a ring of dark grass round the outside edge, or any sinking of the now dead part? Was there once a tree or shrub in that area? Has anything been left on the grass in that area at any time? Has there been a spillage of something in that area? Did the dead patch appear all at once at that size and shape, or did it spread out slowly? What kind of turf does he have - luxury or utility (ordinary) lawn grass? Is the soil sandy or heavy?

The guy's just gone on holiday for 3 weeks but will ask more on his return. I seem to remember mycelium is a bit like cotton wool but this is a powder
He thought the dead patch was due to the conifer but it's the powder he wants to solve.

Its probably been there for years - can't think how a layer of powder could build up through grass otherwise.

I have several conifers in my garden - junipers, norway spruce, black pine. I never had brown patches anywhere. They use water conservatively.

Could it be Fusarium Patch? I don't know whether you get mycelium under the grass with that though.

humm, since the patch is 2x2 sq yards, was there a table there or some other structure; bird bath or something square that blocked out the sunlight and caused the grass to go brown?

I had some strange small round dead patches on my lawn. It puzzled me until I realised that the bee keepers who took away a swarm in a tree had put down their smoke gun in several places, producing neat burnt patches!

Thanks guys, I guess we'll never inow now as my friend put off his holiday to dig the area out, refill it with good soil and re-turfed it.