Just few things to check.
Although wfp brushes themselves cannot scratch glass, brick dust can. I have seen so many wfp users place their brushes bristle down onto brickwork, make sure you havent done that.
Also does the house have any unpainted rendering, or peppledash? It can crumble off around the sides of the window and could get into the bristles.
And finally, try to see this issue from the cusomers prospective. Be firm, dont admit liability and test the brush with himnto show him its impossible to make scratches.
If it completly breaks down, and he is adament that its your fault, offer a refund on the clean and make it clear to him that you are doing it as a goodwill guesture, and that you are not at fault. Make him understand you know how he ciuld be feeling, and its unfortunate but you are not admitting liability.