I had a lady mention this about her plated "gold"/"brass" door letterbox and asked me if I would leave the door. (It only had a small window in and so I buff it with a scrim now)
She said to me "the problem is the stuff they make nowadays is just cheap and nasty" which is essentially often true. She was fine about it.
It's cheap crap which weathers badly. IMO
I would write a letter saying I am not responsible for the cheap materials used by her glazing company and that I use no chemicals in my pure water. I would also state that "some window companies build down to a price and use the cheapest of materials" - also add a paragraph that if the company representative has blamed me and my system then I consider that a slanderous statement and I would like his company name so that "I may seek redress".
You will have firmly stated your case, the householder will not want to get involved and will shut up. If they're really good they'll take it up with the company, but I doubt it ...