Why don't you give her to another local windie who you know will do a good job?
You can send her a text saying
'Dear Mrs Smith, unfortunately I won't be able to clean your windows anymore, but I have arranged for another window cleaner (Fred Smith) who is very professional, to contact you and introduce himself. I am sure he will continue to do a good job on your behalf. I would like to take this opportunity to thank you for your custom.