If someone asks you to clean their windows, they have to accept that there is a risk involved. At the same time, you should have liability insurance to cover you in the eventuality of damage occurring.
Unless the leads are DIY, they should be capable of withstanding being cleaned, otherwise they are not fit for purpose. Can you imagine buying a car, then being told you mustn't clean it in case that causes damage?
Before WFP the only practical way to clean leaded glass was the traditional leather and scrim - dragging a scrim across leads is far more likely to pull them off than swishing them with a soft brush, plus you used to have to rub very hard to polish off the leather marks.
My take on this is the same as the ever recurring "WFP scratched my windows". If a customer accuses you of damaging their leaded windows, tell them you will report it to your insurance company. If the insurance company denies liability that is the end of the matter. That's why you have insurance.
If you're working without Public Liability then you deserve to be sued by your customer