Whether or not cleaning windows with WFP is leaving marks on your customers walls, natural weathering also causes marks and differentiating between what we may cause and what is caused by natural weathering is more often than not impossible. Unfortunately some customers seem to think that their new pvcu window frames/cladding or newly painted rendering should stay pristine much longer than experience tells us that it will, and are too ready to blame us when it starts to get dirty. There's often not much we can do but be reasonable and hope they will be too. But if they don't, all we can do is move on. It is unfortunate that the old lady in your case has 'spread the word.' Many of us have had similar experiences. You're not alone.