I have had a look at Strouts, Mattheck, Phillips etc. notnhing is said about the mode of decay or the pathogenic vigour of X.p. I looked at 'Diseases of Trees and Shrubs' Sinclair et al, an american book which is incredibly thorough. They say that' X. polymorpha is a proven pathogen only of apple. They go on to list species on which they can be found on the butt, including A. saccharinum but not A. pseudoplatanus. Evidence is restricted to USA and Canada. The mode of attack is given in great detail, I can send it if required but it does not say what kind of decay (white, brown, soft) is caused except what is described as an off-white decay of wood beneath a black sheath.
I would be suggesting to the client that although there is no widely published evidence of pathogenesis from X. polymorpha on A. pseudoplatanus, its presence suggests that wood has been killed or made vulnerable by other pathogens and that its near-surface activity can only be complementing the internal weakening of wood by other pathogens and interfering with vascular flows to accelerate the demise.
I would then add that I am not being paid to give a definitive professional view so don't sue me if the tree blows over but you may wish to keep a close eye on it to assess vigour and vitaility (lots of silhouette photos this winter to compare with some next winter). Or pay to get someone else to blame if it goes wrong or to do intrusive investigation. Or reduce knowing that this will accelerate the demise of the whole tree. Or cut losses and fell and replant.
So, having leafed through my library I would say there is not much said definitively about X. p that helps you. Unless anyone else knows something? You sound like the sort who wants to be helpful but again I would suggest the more helpful you are the more it will be construed as expertise that an owner might erroneously rely on. The building insurer, trying to duck out of a claim arising from a catastrophe, might come looking for someone like you to blame. It might count for little that you weren't paid for advice, it is an existing client which could only make the construction of an implied duty of care easier. Gloomy I know but I am just being careful.
ASnd if the tree does get felled, any chance of some pics of the cut stump? Sinclair has ifo that might make the extent of the X. p damage identifiable. Again I can post that info if you want.