The custy is telling you to think on it............she couldnt be any more patronising could she?
Why dont you tell her to swivel on it mate........walk away and get another custy 
Iwitot
why is she being patronising ?
she sounds as though she's telling him very politely that the service she has paid him for is not very good on this occasion and is leaving the ball in his court to come and put it right.
Any decent window cleaner would be straight on the phone to her to appologise and tell her he will be straight back round to clean them again, after all she's been paying him for 4 years.
your attitude is absolutely diabolical to suggest to a customer of 4 years she should go and swivel on it because she has spotted the fact the windows haven,t been cleaned properly.
Thankfully a lot of window cleaners take pride in their work and like to build up a good working relationship with their customers, unfortunately there are some that are just in it for greed and don't care if they provide a good or bad service as long as they get paid.
Eddie D
What amazes me is the fact that you have actually come on here to complain about it instead of going to see this lady who has paid you for the last 4 years to put the job right.
Why write a letter ? and yet you say your not bothered but would like to turn her around, well the solution is simple, do the job properly and she wont complain