I have a builder who done this with his own windows, and I just told him, it might never come off, so its probably about £500 of damages if you get a glazer to replace the glass.
Why customers don't sue these people I don't know, I bet they tell them, your window cleaner will get that off no problem, see ya. Take the money and run. I have another customer that's porch was covered in it. They may as well as just surfaced that over too, huge chuncks like they didn't give a rats crap.