I would not be worried about this, I live by the Pilkingtons Factory and a few of my mates work there, I have asked them in the past if it is possible for wfp to damage the units or even scratch the glass, because my jets are inside the brush head and the only contact with glass is plastic, the chances I scratch their glass is 1 in 10 million.
As for the seals, to make the sealed unit work, Argon Gas is pumped in between the 2 panes of glass and then they are sealed, for the insides to condense up with water is when the seal is broken and the Argon Gas has escaped.
The main reason this happens is because of movement to the house betwen daytime and night time, also when its hot and cold.
The life of a sealed unit is estimated at 10 years, so if one of her panes is 9 year old, then clearly its at it maximum life cycle.
I would tell her this and say that your would welcome her to take court action, I would mention that the FEDERATION of WINDOW CLEANERS LEGAL DEPARTMENT are backing you.
I would also point out that if she insists on telling people that you caused this damage without evidence is slander and if she carrys on then you will have no choise but to seek compensation though the courts.