I think it is fair game, you were not setting out to poach the work, and presumably this window cleaner is not one of your collegues, you did no wrong.
If it is a window cleaner I know, then I will personally not take the work, but outside of that, well, that's business I'm afraid.
With WFP, one of it's many strong points is the fact that you can get at windows that simply cannot be done trad, plus the fact that you can also thoroughly wash down the likes of frames and doors...big selling points, trad cleaners will lose out more and more as the use of WFP becomes more and more accepted by the general public.
There are of course occasions when a WFP window cleaner will lose out to a trad cleaner, but it is now more likely to be the other way around.
Ian