I have never cleaned a window in my life using a WFP. So unable to comment on the cause of the scratching. I do however, have a lot of experience with customers, who sometimes try it on, then, then there are those who are genuine.
Firstly, I think you are taking this too personally and you are allowing yourself to be wound up by the customer because of his general demeanour and attitude! Would you be the same if the customer was a gorgeous girl living on her own and was always pleasant? My resolution would be, as follows, be pleasant, tell him you will get it sorted, whether it’s £50 or £150! just write it off, claim it as a tax deductible expense on you books and forget it.
If you pursue the matter and he wins, which I can tell you he almost certainly will because your insurance would pay out rather than start an expensive legal fight over what is a paltry sum. Then, should you lose, you need to consider if you have an excess on your policy to pay (if so how much) then probably a rise in Premium next year. In my opinion just not worth the agro over the sums involved. But ultimately your decision.
Kev