Similar to daz's answer.
The customer is simply amazed at how clean the windows are lasting, which we know is the case with wfp. It can also be the case if trad cleaning depending upon the products used. For instance i used gg4 from titan and it doesnt leave a sticky soapy residue which allows the dirt particles to stick to the windows easily.
Obviously it also depends where the customers live, as coastal areas wont be so lucky, but my customers are informed that their windows will stay cleaner for longer because of this very fact.
So just be honest with her and educate her rather than pretending it some sort of magic water (which some believe esp if its the powerfull 000 tds type)
(For the record, hot (over 60 degree) water will soften concrete splatter to allow easy removal)