If you think she is giving you a rubber ear, then stop cleaning her windows. Explain politely the cost of traveling 20 miles to chase payment and how that effects your business, and without prompt payment it is not worth your while.
Dont make it cound like you are giving up but let her know that if she wants her windows cleaned again then she must settle her bill before you will give her a booking date.
I have had a couple of customers that have seperated from there spouce or a very unfortunate death in there family, and my thoughts and condolences go out to people in these situations. However you still have to carry on working, and they must still pay there gas, rent, electricity and so forth. Many fortunately aren't a problem, but some do take this different from others and I have have trouble with a minority, similar to your story and ended up dropping them after some calls and failed visits.
hope you get it sorted
gerard