I understand your frustration but to call him a cowboy without even knowing him!
Like someone has said, he may of been out canvassing and one of your customers asked him how much and he gave them a price which was cheaper and they decided to go with him and drop you!
I am always friendly to my customers and make time to chat to them, that way they see you as more than 'just' the window cleaner and are less likely to drop you!
Likewise when I am canvassing if they have a window cleaner I wont give them a quote but leave them with a flyer incase he stops coming or they happen to drop him for any reason, this may cost me houses but I think window cleaners should help each other and not try to pinch work off each other